Fluid brake control system

Abstract

A fluid brake control system is disclosed for governing application and release of brakes of an interchange vehicle having a brake control pipe, an auxiliary fluid reservoir, a fluid brake cylinder, and a brake control device sensing changes in fluid pressure in the brake pipe for at times governing application of fluid to the brake cylinder from the auxiliary reservoir and at other times for releasing fluid from the brake cylinder. Additional first and second volume chambers are provided that are proportionally smaller in volume than the auxiliary reservoir and brake cylinder displacement volumes respectively. Valve governed passageways are provided for normally charging the first volume chamber from the brake pipe and for preventing fluid flow from the auxiliary reservoir to the first volume chamber. The second volume chamber is charged from the first volume chamber upon sensing a reduction of pressure in the brake pipe at a rate to reduce pressure in the first volume chamber proportional to a service rate of reduction in the brake pipe, and relay control apparatus is provided for sensing a difference in fluid pressure between the brake cylinder pressure and pressure in the second volume chamber for governing the flow of fluid from the auxiliary reservoir to the brake cylinder. An emergency application is obtained by the use of sequenced proportioning valves wherein the brake cylinder is pressurized first from the auxiliary reservoir and then from an emergency reservoir. Apparatus is provided by which the second volume chamber can have its effective volume varied in accordance with the weight of the vehicle.

What is claimed is: 
     
       1. A fluid brake control system for governing application and release of brakes of an interchange vehicle having a brake control pipe, .[.an auxiliary.]. .Iadd.a first .Iaddend.fluid reservoir, a fluid brake cylinder, and a brake control device sensing changes in fluid pressure in the brake pipe for at times governing application of fluid to the brake cylinder from .[.the auxiliary.]. .Iadd.said first .Iaddend.reservoir and at other times for releasing fluid from the brake cylinder wherein improvements in the brake control device .[.comprises.]..Iadd.comprise.Iaddend.; (a) first and second volume chambers .[.bath proportionally substantially.]. .Iadd.both proportionately .Iaddend.smaller in volume than the respective .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and brake cylinder displacement volumes,   (b) means including the brake control device for charging the first volume chamber from the brake pipe and for preventing fluid flow from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir to the first volume chamber,   (c) means including the brake control device for charging the second volume chamber from the first volume chamber upon sensing a reduction of pressure in the brake pipe, and   (d) relay valve means sensing a difference in fluid pressure between brake cylinder pressure and pressure in the second volume chamber for governing .[.flow of fluid from the auxiliary reservoir to.]. .Iadd.the degree of braking force applied by .Iaddend.the brake cylinder.   
     
     
       2. A fluid brake control system according to claim 1 wherein the ratio of the sum of the volumes of the first and second chambers to the volume of the first chamber is approximately the same as the ratio of the sum of the volumes of the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and the brake cylinder displacement volume to the volume of the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir. 
     
     
       3. A fluid brake control system according to claim 2 wherein choke means is provided to cause reduction in pressure in the first volume chamber .Iadd.during charging of the second volume chamber, .Iaddend.at a rate comparable to a service rate of reduction of pressure in the brake pipe. 
     
     
       4. A fluid brake control system according to claim 1 wherein means including a proportioning valve is provided for limiting pressure reduction in the first volume chamber .Iadd.during charging of the second volume chamber, .Iaddend.to a fixed percentage of a reference pressure. 
     
     
       5. A fluid brake control system according to claim 4 wherein the reference pressure is the pressure in .[.an emergency.]. .Iadd.a second .Iaddend.reservoir. 
     
     
       6. A fluid brake control system according to claim 1 wherein service cut-off and proportioning valve means .[.is.]. .Iadd.are .Iaddend.provided for obtaining an emergency brake application by pressurizing the brake cylinder first from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and then from .[.an emergency.]. .Iadd.a second .Iaddend.reservoir in fixed proportions. 
     
     
       7. A fluid brake control system according to claim 6 wherein the proportioning valve means has first and second valves governing charging of the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and .[.emergency.]. .Iadd.second .Iaddend.reservoir, respectively, and proportioning piston means for first opening the first valve while maintaining the second valve closed, secondly opening the second valve while maintaining the first valve closed and finally closing both valves when the brake cylinder has been pressurized to .[.full.]. .Iadd.an .Iaddend.emergency pressure. 
     
     
       8. A fluid brake control system according to claim 7 wherein means is provided for rendering the proportioning means effective to open only the first valve for a service brake application. 
     
     
       9. A fluid brake control system according to claim 6 wherein the cut-off valve means comprises; (a) a differential abutment subject to actuation by opposing .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and brake cylinder pressures,   (b) a first control valve means being opened by movement of the abutment in one direction for delivering fluid from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir to the brake cylinder, and   (c) a second control valve means being opened by movement of the abutment in the opposite direction when the brake cylinder is charged to substantially full service pressure for delivering fluid from the .[.emergency.]. .Iadd.second .Iaddend.reservoir to the brake cylinder.   
     
     
       10. A fluid brake control system according to claim 1 wherein means is provided for varying the effective volume of the second volume chamber in accordance with the weight of the vehicle. 
     
     
       11. A fluid brake control system according to claim 10 wherein proportioning means including a normally open valve is provided for permitting flow of fluid from the first volume chamber to the second volume chamber only until pressure in the first volume chamber is reduced to a fixed proportion relative to a reference pressure. 
     
     
       12. A fluid brake control system according to claim 11 wherein the proportioning means comprises a stepped piston for actuating the normally open valve, one face of the piston being subject to pressure of the first volume chamber and an opposite face of the piston being subject to the reference pressure. 
     
     
       13. A fluid brake control system according to claim 12 wherein the reference pressure is pressure of .[.an emergency.]. .Iadd.a second .Iaddend.reservoir. 
     
     
       14. A fluid brake control system according to claim 13 wherein the proportioning means is effective when its stepped piston is actuated to a position closing the normally open valve to prevent further reduction of pressure in the first volume chamber for closing a reference pressure control valve for maintaining fixed .[.emergency.]. .Iadd.reference pressure from the second .Iaddend.reservoir .[.reference pressure.]. acting on the piston, irrespective of subsequent reduction of pressure in the .[.emergency.]. .Iadd.second .Iaddend.reservoir. 
     
     
       15. A fluid brake control system according to claim 1 wherein service limiting valve means is provided for sensing a drop in brake pipe pressure and delivering fluid from the brake pipe to the brake cylinder only until pressure in the .[.second volume chamber.]. .Iadd.brake cylinder .Iaddend.reaches a fixed level. 
     
     
       16. A fluid brake control system according to claim 1 wherein continuous quick service valve means is provided for sensing a drop in brake pipe pressure and for venting fluid periodically from the brake pipe and from a quick action chamber as long as the brake pipe pressure continues to be reduced independent of the associated brake control device. 
     
     
       17. A fluid brake control system according to claim 1 wherein emergency valve means is provided having a differential abutment sensitive to differences in brake pipe and quick action chamber pressures for sensing an emergency rate of reduction of brake pipe pressure and in accordance therewith for actuating vent valve means to vent fluid from the brake pipe at an emergency rate. 
     
     
       18. A fluid brake control system according to claim 1 wherein means is provided for sensing an increase in brake pipe pressure relative to quick action chamber pressure for delivering fluid from .[.the brake cylinder and the emergency.]. .Iadd.a second .Iaddend.reservoir to the brake pipe to provide accelerated release of brakes of other vehicles of a train. 
     
     
       19. A fluid brake control system according to claim 1 wherein means is provided for releasing fluid from the brake cylinder in response to manual actuation of a release valve. 
     
     
       20. A fluid brake control system for governing application and release of brakes of an interchange vehicle having a brake control pipe, .[.an auxiliary.]. .Iadd.a first .Iaddend.reservoir, .[.an emergency.]. .Iadd.a second .Iaddend.reservoir, a fluid brake cylinder, and a brake control device contained in a housing for sensing changes in fluid pressure in the brake pipe for at times governing application of fluid to the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and the .[.emergency.]. .Iadd.second .Iaddend.reservoir and at other times for releasing fluid from the brake cylinder wherein improvements in the brake control device comprise; (a) first and second fluid volume chambers external of the brake control device connected by passages to ports of the brake control device,   (b) covers for the ports associated with the passages, and   (c) means for connecting .Iadd.either .Iaddend.the volume chambers .[.and.]. .Iadd.or .Iaddend.the covers .[.alternately.]. to the brake control device .[.and for at times providing.]. .Iadd.for providing, respectively, .Iaddend.a full service brake application by rendering the brake control device operable to govern brake cylinder pressure in accordance with the equalizing of the brake cylinder pressure with pressure in .Iadd.at least one of .Iaddend.the volume chambers, .[.and at other times.]. .Iadd.or .Iaddend.in accordance with the equalizing of the brake cylinder pressure with pressure in the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir .[.respectively.]..   
     
     
       21. A fluid brake control system according to claim 20 wherein when the volume chambers are connected to the brake control device, means is provided including a proportioning valve device for preventing flow of fluid from the first volume chamber to the second volume chamber when the ratio of pressures of the .[.emergency.]. .Iadd.second .Iaddend.reservoir and the first volume chamber is below a fixed ratio. 
     
     
       22. A fluid brake control system for governing application and release of brakes of an interchange vehicle having a brake control pipe, .[.an auxiliary fluid.]. .Iadd.a first .Iaddend.reservoir, .[.an emergency fluid.]. .Iadd.a second .Iaddend.reservoir, a fluid brake cylinder and control means sensing changes in fluid pressure in the brake pipe for at times governing application of fluid to the brake cylinder from the reservoirs, and at other times for releasing fluid from the brake cylinders and recharging the reservoirs, wherein improvements in the brake control device comprise; (a) application control means including proportioning valve means governed by brake pipe pressure and effective upon sensing an emergency rate of reduction in the brake pipe to first charge the brake cylinder .[.to substantially full service pressure.]. from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and then .Iadd.to .Iaddend.shift charge of the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir to the .[.emergency.]. .Iadd.second .Iaddend.reservoir,   (b) the proportioning valve means being effective to lap the charging of the brake cylinder from the .[.emergency.]. .Iadd.second .Iaddend.reservoir when the brake cylinder has been charged to a pressure that is a fixed percentage above .[.full.]. service braking pressure.   
     
     
       23. A fluid brake control system according to claim 22 wherein the fixed percentage of .[.full.]. service braking pressure is approximately 120%. 
     
     
       24. A fluid brake control system according to claim 22 wherein the proportioning valve means comprises stepped piston means for sensing when a force on one face of the piston means .[.of.]. .Iadd.due to .Iaddend.brake cylinder pressure substantially equals a total force on an opposite and larger face of the piston means .[.generated by.]. .Iadd.due to .Iaddend.pressure from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir .Iadd.and thereafter .Iaddend.for lapping the charging of the brake cylinder. 
     
     
       25. A fluid brake control system according to claim 24 wherein the stepped piston means has an associated chamber subject to brake pipe pressure for preventing operation of the proportioning valve means during a service brake application. 
     
     
       26. A fluid brake control system according to claim 22 wherein the application control means comprises cut-off valve means having a first control valve means for governing application of fluid to the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir, and having a second control valve means for governing application of fluid to the brake cylinder from the .[.emergency.]. .Iadd.second .Iaddend.reservoir. 
     
     
       27. A fluid brake control system according to claim 26 wherein the cut-off valve means comprises differential abutment means subject to opposing .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and brake cylinder pressures for operating the first and second control valves of the cut-off valve means. 
     
     
       28. A fluid brake control system according to claim 27 wherein the first and second control valves are poppet type valves disposed on opposite sides of the abutment means in normally closed positions, subject to actuation to open positions upon movement of the abutment means respectively in one direction or another. 
     
     
       29. A fluid brake control system according to claim 27 wherein; (a) means is provided including the first control valve means for delivering fluid to the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir when the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir pressure is materially higher than the brake cylinder pressure, and   (b) means is provided including the second control valve means for delivering fluid to the brake cylinder from the .[.emergency.]. .Iadd.second .Iaddend.reservoir when .Iadd.the .Iaddend.difference in pressure between the brake cylinder pressure and the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir pressure is materially reduced.   
     
     
       30. A fluid brake control system for governing application and release of brakes of an interchange vehicle having a brake control pipe, .[.an auxiliary.]. .Iadd.first .Iaddend.reservoir, .[.an emergency.]. .Iadd.a second .Iaddend.reservoir, a fluid brake cylinder, and a brake control device for sensing changes in fluid pressure in the brake pipe for at times governing application of fluid to the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and .[.emergency.]. .Iadd.second .Iaddend.reservoir and at other times for releasing fluid from the brake cylinder, wherein improvements in the brake control device comprise: (a) a universal control valve device,   (b) means for adapting the universal control valve device to either of two alternative modes of operation,   (c) the adapting means in a first state governing operation of the brake cylinder to a braking pressure determined by the relative fluid pressures in the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and in the brake cylinder,   (d) the adapting means in a second state operating the brake cylinder to a braking pressure determined by the relative fluid pressures in two volume chambers having volumes substantially less than and proportional to those of the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and brake cylinder displacement volumes respectively.   
     
     
       31. A fluid brake control system according to claim 30 wherein the adapting means comprises certain fluid passages of the universal control valve device adapted to be selectively restricted or connected with other passages in accordance with which of the alternate modes of operation is selected. 
     
     
       32. A fluid brake control system according to claim 31 wherein the certain fluid passages of the universal control valve device are in part restricted and in part connected together in the first state of the adapting means. 
     
     
       33. A fluid brake control system according to claim 31 wherein the certain fluid passages are selectively connected to passages leading to the volume chambers respectively when the adapting means is in the second state. 
     
     
       34. A fluid brake control system according to claim 32 wherein the adapting means comprises a detachable housing cover for selectively connecting the certain passages of the universal control valve device together when the adapting means is in the first state. 
     
     
       35. A fluid brake control system according to claim 33 wherein the adapting means in its second state comprises first and second volume chambers, both .[.proportionally substantially.]. .Iadd.proportionately .Iaddend.smaller in volume than the respective .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and brake cylinder volumes, selectively connected with the certain passages of the universal control valve device. 
     
     
       36. A fluid brake control system for governing application and release of brakes of an interchange vehicle having a brake control pipe, .[.an auxiliary fluid.]. .Iadd.a first .Iaddend.reservoir, a fluid brake cylinder, and a brake control device .[.having emergency and service portions secured to a pipe bracket.]. for sensing changes in fluid pressure in the brake pipe and for at times governing application of fluid to the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir, and at other times for releasing fluid from the brake cylinder, wherein an improved .[.service portion.]. .Iadd.brake control device .Iaddend.comprises; (a) application, .[.charging,.]. and release means being poppet type valves respectively disposed partly on one side and partly on an opposite side of a differential abutment and operably associated with the abutment for respectively .[.applying.]. .Iadd.controlling application of .Iaddend.fluid pressure to the brake cylinder.[., charging the auxiliary reservoir,.]. and .[.releasing.]. .Iadd.release of .Iaddend.fluid from the brake cylinder in accordance with the .[.abutment.]. .Iadd.abutment's .Iaddend.sensing differences in pressure of fluid in the brake pipe as compared to a .Iadd.first .Iaddend.reference fluid pressure, and   (b) relay valve means for governing pressure in the brake cylinder in accordance with sensing brake cylinder pressure relative to a .Iadd.second .Iaddend.reference pressure.   
     
     
       37. A fluid brake control system according to claim 36 wherein means is provided for selectively rendering the relay valve means effective or ineffective for governing pressure in the brake cylinder. 
     
     
       38. A fluid brake control system according to claim 36 wherein the .[.service portion.]. .Iadd.control device .Iaddend.comprises limiting valve means for limiting delivery of fluid from the brake pipe to the brake cylinder during a brake application in accordance with brake cylinder pressure. 
     
     
       39. A fluid brake control system according to claim 38 wherein the limiting valve means comprises a poppet valve that is normally open to permit delivery of brake pipe fluid to the brake cylinder but is closed when pressure in the brake cylinder is built up to a predetermined value. 
     
     
       40. A fluid brake control system according to claim 36 wherein the .[.service portion.]. .Iadd.control device .Iaddend.comprises brake cylinder release means having poppet type valves respectively disposed partly on one side and partly on the opposite side of a differential abutment and operably associated therewith, the differential abutment being adapted to compare spring biasing pressure on one side of the abutment with fluid pressure admitted to a pressure chamber on the other side of the abutment in response to a manual release designation for closing a brake cylinder application passage and exhausting fluid from the brake cylinder. 
     
     
       41. A fluid brake control system according to claim 40 wherein .Iadd.the control device comprises an application passage leading to the brake cylinder and .Iaddend.the brake cylinder release means comprises a normally closed poppet type valve that is opened in response to the manual release designation for admitting fluid from the application passage to the pressure chamber for maintaining the brake cylinder release means actuated as long as the application passage is pressurized. 
     
     
       42. A fluid brake control system according to claim 36 wherein the .[.service portion has.]. .Iadd.control device comprises an application passage leading to the brake cylinder and .Iaddend.reset valve means for permitting reduction of pressure in .[.an.]. .Iadd.the .Iaddend.application passage substantially to pressure in the brake pipe when pressure in the brake pipe is substantially greater than pressure in the brake cylinder. 
     
     
       43. A fluid brake control system according to claim 42 wherein the reset valve means has a normally closed poppet type valve that is opened upon sensing a greater pressure in the brake pipe than pressure in the brake cylinder for permitting reduction of pressure in the application passage to substantially correspond to the pressure in the brake pipe. 
     
     
       44. A fluid brake control system for governing application and release of brakes of an interchange vehicle having a brake control pipe, .[.auxiliary and emergency.]. .Iadd.first and second .Iaddend.reservoirs, a fluid brake cylinder, and a brake control device having emergency and service portions secured to a pipe bracket for sensing distinctively service and emergency rates of change respectively in fluid pressure in the brake pipe for at times selectively governing application of fluid to the brake cylinder and at other times for releasing fluid from the brake cylinder, wherein an improved emergency portion of the brake control device comprises; (a) application control means including proportioning valve means governed by brake pipe pressure and effective upon sensing an emergency rate of reduction in the brake pipe to first charge the brake cylinder .[.to substantially full service pressure.]. from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and then .Iadd.to .Iaddend.shift charge of the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir to the .[.emergency.]. .Iadd.second .Iaddend.reservoir,   (b) the proportioning valve means being effective to lap the charging of the brake cylinder by the .[.emergency.]. .Iadd.second .Iaddend.reservoir when the brake cylinder has been charged to a pressure that is a fixed percentage above .[.full.]. service braking pressure.   
     
     
       45. A fluid brake control system according to claim 44 wherein the proportioning valve means comprises a proportioning device having a pressure chamber connected to the brake pipe, the pressurization of which during a service brake application renders the proportioning valve means inoperable to shift the charging of the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir to the .[.emergency.]. .Iadd.second .Iaddend.reservoir. 
     
     
       46. A fluid brake control system according to claim 44 wherein the proportioning valve means comprises a cut-off valve device having, (a) a differential abutment for sensing differences in pressure of chambers on opposite sides of the abutment subject to application pressure from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and brake cylinder pressure respectively,   (b) poppet type valve means .[.disposed on opposite sides of the abutment.].  subject to actuation by the abutment for governing the charging of the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir and from the .[.emergency.]. .Iadd.second .Iaddend.reservoir respectively.   
     
     
       47. A fluid brake control system according to claim 46, wherein the poppet type valve means includes .[.auxiliary and emergency.]. .Iadd.two .Iaddend.poppet valves disposed on opposite sides of the abutment for governing flow of fluid from the .[.auxiliary and emergency.]. .Iadd.first and second .Iaddend.reservoirs respectively, wherein actuation of the abutment in one direction charges the brake cylinder from the .[.auxiliary.]. .Iadd.first .Iaddend.reservoir through .[.the auxiliary.]. .Iadd.one .Iaddend.poppet valve, and actuation of the abutment in the opposite direction charges the brake cylinder from the .[.emergency.]. .Iadd.second .Iaddend.reservoir through the .[.emergency.]. .Iadd.other .Iaddend.poppet valve. 
     
     
       48. A fluid brake control system according to claim 47 wherein the .[.auxiliary and emergency.]. poppet valves charge the brake cylinder through the brake cylinder pressure chamber of the cut-off valve device. 
     
     
       49. A fluid brake control system according to claim 48 wherein a passage is provided for delivering fluid to the brake cylinder through the brake cylinder chamber of the cut-off device from the .[.emergency.]. .Iadd.second .Iaddend.reservoir at a restricted rate, irrespective of the condition of the .[.emergency.]. .Iadd.other .Iaddend.poppet valve. 
     
     
       50. A fluid brake control system according to claim 44 wherein the emergency portion comprises a combined quick service and accelerated release valve means comprising (a) a valve device having a differential abutment sensing differences in pressure of chambers subject to pressure of the brake pipe on one side of the abutment and pressure of a quick action chamber on the other side of the abutment, and   (b) a pair of poppet type valve means operably connected to the abutment on one side thereof and adapted to be actuated sequentially in response to actuation of the abutment in one direction to alternately initiate the venting of fluid from the brake pipe and the quick action chamber in response to the valve device sensing reduction of pressure in the brake pipe relative to pressure in the quick action chamber.   
     
     
       51. A fluid brake control system according to claim 50 wherein the valve device has accelerated release valve means on the other side of the abutment comprising (a) poppet type valve means operably connected to the abutment for connecting the .[.emergency.]. .Iadd.second .Iaddend.reservoir to the brake pipe when pressure in the brake pipe is higher than pressure in the quick action chamber so as to move the abutment in the opposite direction, and   (b) a spring biased piston on said other side of the abutment normally maintained separated from the abutment by quick action chamber pressure,   (c) the spring bias piston being rendered effective by depletion of pressure in the quick action chamber to load the abutment for preventing premature connection of the .[.emergency.]. .Iadd.second .Iaddend.reservoir to the brake pipe for accelerated release after an emergency brake application. .Iadd. 52. A fluid brake control system according to claim 1 wherein said means for charging the first and second volume chambers comprises:   a differential abutment for sensing differences in pressure of chambers on opposite sides of the abutment subject respectively to brake pipe pressure and pressure from said first volume chamber, and   poppet valve means subject to actuation by the abutment for governing charging of the first volume chamber and the first reservoir from the brake pipe, and for governing charging of the second volume chamber from the first volume chamber. .Iaddend..Iadd. 53. A fluid brake control system according to claim 52 wherein choke means is provided to cause reduction in pressure in the first volume chamber during charging of the second volume chamber, at a rate comparable to a service rate of reduction of pressure in the brake pipe. .Iaddend..Iadd. 54. A fluid brake control system according to claim 52 wherein means including a proportioning valve is provided for limiting pressure reduction in the first volume chamber during charging of the second volume chamber, to a fixed percentage of a reference pressure. .Iaddend. .Iadd. 55. A fluid brake control system according to claim 54 wherein the reference pressure is the pressure in a second reservoir. .Iaddend..Iadd. 56. A fluid brake control system according to claim 1, wherein said relay valve means comprises:   a differential abutment for sensing differences in pressure of chambers on opposite sides of the abutment subject respectively to brake cylinder pressure and pressure from said second volume chamber, and   poppet valve means subject to actuation by the abutment for governing flow of fluid from the first reservoir to the brake cylinder. .Iaddend..Iadd. 57. A fluid brake control system according to claim 56, further comprising:   a quick service volume;   second poppet valve means subject to actuation by the abutment for governing flow of fluid from the brake pipe to the quick service volume; and   quick service limiting valve means for delivering fluid from the quick service volume to the brake cylinder until pressure in the brake cylinder reaches a fixed level. .Iaddend..Iadd. 58. A fluid brake control system according to claim 7, wherein said proportioning piston is a stepped piston having a first, smaller face and a second, larger face oriented opposite to said first face, said first face being exposed to brake cylinder pressure and said second face being exposed to pressure of fluid flowing from said relay valve means. .Iaddend. .Iadd. 59. A fluid brake control system according to claim 58, wherein said piston comprises a third face oriented opposite to said second face, said third face being exposed to brake pipe pressure, whereby said piston is biased to open only said first valve during a service brake application. .Iaddend..Iadd. 60. A fluid brake control system according to claim 58, wherein said first smaller face is sized relative to said second, larger face so that during an emergency application the piston moves to close both the first and second valves when the pressure in the brake cylinder is a fixed percentage above service braking pressure. .Iaddend..Iadd. 61. A fluid brake control system according to claim 60 wherein the fixed percentage of service braking pressure is approximately 120%. .Iaddend. .Iadd. 62. A fluid brake control system according to claim 9, further comprising a passage around the second control valve means, for delivering fluid to the brake cylinder from the second reservoir at a restricted rate, regardless of the position of the first control valve means. .Iaddend..Iadd. 63. A fluid brake control system according to claim 1 wherein proportioning means including a normally open valve is provided for permitting flow of fluid from the first volume chamber to the second volume chamber only until pressure in the first volume chamber is reduced to a fixed proportion relative to a reference pressure. .Iaddend..Iadd. 64. A fluid brake control system according to claim 63 wherein the proportioning means comprises a stepped piston for actuating the normally open valve, one face of the piston being subject to pressure of the first volume chamber and an opposite face of the piston being subject to the reference pressure. .Iaddend..Iadd. 65. A fluid brake control system according to claim 64 wherein the reference pressure is pressure of a second reservoir. .Iaddend..Iadd. 66. A fluid brake control system according to claim 65 wherein the proportioning means is effective when its stepped piston is actuated to a position closing the normally open valve to prevent further reduction of pressure in the first volume chamber, for closing a reference pressure control valve for maintaining fixed reference pressure from the second reservoir acting on the piston, irrespective of subsequent reduction of pressure in the second reservoir. .Iaddend..Iadd. 67. A fluid brake control system according to claim 16, wherein said continuous quick service valve means comprises:   a differential abutment for sensing differences in pressure of chambers on opposite sides of the abutment subject respectively to brake pipe pressure and pressure from the quick action chamber,   first poppet valve means subject to actuation by the abutment for governing release of fluid under pressure from the brake pipe to atmosphere and subsequently release of fluid under pressure from the quick action chamber to atmosphere. .Iaddend..Iadd. 68. A fluid brake control system according to claim 67, wherein the continuous quick service valve also comprises:   second poppet valve means subject to actuation by the abutment for connecting a second reservoir to the brake pipe when the pressure in the brake pipe is sufficiently higher than the pressure in the quick action chamber;   a spring biased piston operatively associated with the abutment and normally biased out of contact with the abutment by pressure in the quick action chamber, whereby depletion of pressure in the quick action chamber causes said piston to load the abutment to prevent premature opening of the second poppet valve means; further comprising an emergency application and stability valve means including:   a further differential abutment for sensing differences in pressures on opposite sides of the further abutment subject respectively to brake pipe pressure and pressure from the quick action chamber,   third poppet valve means subject to actuation by the further abutment to curtail flow of fluid from the brake pipe to the quick action chamber upon the occurrence of an emergency rate of reduction in brake pipe pressure;   fourth poppet valve means subject to actuation by the further abutment to curtail flow of fluid from the quick action chamber via the first poppet valve means to atmosphere upon the occurrence of an emergency rate of reduction in brake pipe pressure; and   fifth poppet valve means subject to actuation by the further abutment to direct flow of fluid from the quick action chamber to actuate the vent valve means upon the occurrence of an emergency rate of reduction in the brake pipe pressure. .Iaddend..Iadd. 69. A fluid brake control system according to claim 17 wherein the emergency valve means comprises:   a differential abutment for sensing differences in pressures on opposite sides of the abutment subject respectively to brake pipe pressure and pressure from the quick action chamber;   first poppet valve means subject to actuation by the abutment to curtail flow of fluid from the brake pipe to the quick action chamber upon the occurrence of an emergency rate of reduction in brake pipe pressure;   second poppet valve means subject to actuation by the abutment to curtail flow of fluid from the quick action chamber to atmosphere upon occurrence of an emergency rate of reduction in brake pipe pressure; and   third poppet valve means subject to actuation by the abutment to direct flow of fluid from the quick action chamber to actuate the vent valve means upon the occurrence of an emergency rate of reduction in brake pipe   
     
     
        pressure. .Iaddend..Iadd. 70.  A fluid brake control system according to claim 26, further comprising a passage around said second control valve means, for delivering fluid to the brake cylinder from the second reservoir at a restricted rate, regardless of the position of the first control valve means. .Iaddend..Iadd. 71. A fluid brake control system according to claim 24, wherein said stepped piston has a first, smaller face and a second, larger face oriented opposite to said first face, said first face being exposed to brake cylinder pressure and said second face being exposed to pressure of fluid flowing from the first reservoir. .Iaddend..Iadd. 72. A fluid brake control system according to claim 71, wherein said piston comprises a third face oriented opposite to said second face, said third face being exposed to brake pipe pressure, whereby said piston is biased to permit charging of the brake cylinder only from the first reservoir during a service brake application. .Iaddend. .Iadd. 73. A fluid brake control system according to claim 24, wherein the proportioning valve means comprises first and second valves governing charging of the brake cylinder from the first and second reservoirs, respectively, said valves being activated by said stepped piston. .Iaddend..Iadd. 74. A fluid brake control system according to claim 36, further comprising a poppet type charging valve means operatively associated with the abutment for charging the first reservoir. .Iaddend..Iadd. 75. A fluid brake control system according to claim 36, wherein said first and second reference pressures are unequal. .Iaddend..Iadd. 76. A fluid brake control system according to claim 36, wherein service cut-off and proportioning valve means are provided for obtaining an emergency brake application by pressurizing the brake cylinder first from the first reservoir and then from a second reservoir in fixed proportions. .Iaddend..Iadd. 77. A fluid brake control system according to claim 76 wherein the proportioning valve means has first and second valves governing charging of the brake cylinder from the first reservoir and second reservoir, respectively, and proportioning piston means for first opening the first valve while maintaining the second valve closed, secondly opening the second valve while maintaining the first valve closed and finally closing both valves when the brake cylinder has been pressurized to the pressure in the second reservoir. .Iaddend..Iadd. 78. A fluid brake control system according to claim 77 wherein means is provided for rendering the proportioning means effective to open only the first valve for a service brake application. .Iaddend. .Iadd. 79. A fluid brake control system according to claim 77, wherein said proportioning piston is a stepped piston having a first, smaller face and a second, larger face oriented opposite to said first face, said first face being exposed to brake cylinder pressure and said second face being exposed to pressure of fluid flowing from said relay valve means. .Iaddend..Iadd. 80. A fluid brake control system according to claim 79, wherein said piston comprises a third face oriented opposite to said second face, said third face being exposed to brake pipe pressure, whereby said piston is biased to open only said first valve during a service brake application. .Iaddend..Iadd. 81. A fluid brake control system according to claim 79, wherein said first smaller face is sized relative to said second, larger face so that during an emergency application the piston moves to close both the first and second valves when the pressure in the brake cylinder is a fixed percentage above service braking pressure. .Iaddend..Iadd. 82. A fluid brake control system according to claim 81 wherein the fixed percentage of service braking pressure is approximately 120%. .Iaddend..Iadd. 83. A fluid brake control system according to claim 76 wherein the cut-off valve means comprises: a differential abutment subject to actuation by opposing first reservoir and brake cylinder pressures,   a first control valve means being opened by movement of the abutment in one direction for delivering fluid from the first reservoir to the brake cylinder, and   a second control valve means being opened by movement of the abutment in the opposite direction when the brake cylinder is charged to substantially full service pressure for delivering fluid from the second reservoir to the brake cylinder. .Iaddend..Iadd. 84. A fluid brake control system according to claim 83 further comprising a passage around the second control valve means, for delivering fluid to the brake cylinder from the second reservoir at a restricted rate, regardless of the position of the first control valve means. .Iaddend..Iadd. 85. A fluid brake control system according to claim 36, wherein continuous quick service valve means is provided for sensing a drop in brake pipe pressure and for venting fluid periodically from the brake pipe and from a quick action chamber as long as the brake pipe pressure continues to be reduced independent of the associated brake control device. .Iaddend..Iadd. 86. A fluid brake control system according to claim 85, wherein said continuous quick service valve means comprises:   a differential abutment for sensing differences in pressure of chambers on opposite sides of the abutment subject respectively to brake pipe pressure and pressure from the quick action chamber,   first poppet valve means subject to actuation by the abutment for governing release of fluid under pressure from the brake pipe to atmosphere and subsequently release of fluid under pressure from the quick action chamber to atmosphere. .Iaddend..Iadd. 87. A fluid brake control system according to claim 86, wherein the continuous quick service valve also comprises:   second poppet valve means subject to actuation by the abutment for connecting a second reservoir to the brake pipe when the pressure in the brake pipe is sufficiently higher than the pressure in the quick action chamber;   a spring biasing piston operatively associated with the abutment and normally biased out of contact with the abutment by pressure in the quick action chamber, whereby depletion of pressure in the quick action chamber causes said piston to load the abutment to prevent premature opening of the second poppet valve means; further comprising an emergency application and stability valve means including:   a further differential abutment for sensing differences in pressures on opposite sides of the further abutment subject respectively to brake pipe pressure and pressure from the quick action chamber,   third poppet valve means subject to actuation by the further abutment to curtail flow of fluid from the brake pipe to the quick action chamber upon the occurrence of an emergency rate of reduction in brake pipe pressure;   fourth poppet valve means subject to actuation by the further abutment to curtail flow of fluid from the quick action chamber via the first poppet valve means to atmosphere upon the occurrence of an emergency rate of reduction in brake pipe pressure; and   fifth poppet valve means subject to actuation by the further abutment to direct flow of fluid from the quick action chamber to actuate the vent valve means upon the occurrence of an emergency rate of reduction in the brake pipe pressure. .Iaddend..Iadd. 88. A fluid brake control system according to claim 36 wherein emergency valve means is provided having a differential abutment sensitive to differences in brake pipe and quick action chamber pressures for sensing an emergency rate of reduction of brake pipe pressure and in accordance therewith for actuating vent valve means to vent fluid from the brake pipe at an emergency rate. .Iaddend..Iadd. 89. A fluid brake control system according to claim 88 wherein the emergency valve means comprises:   a differential abutment for sensing differences in pressures on opposite sides of the abutment subject respectively to brake pipe pressure and pressure from the quick action chamber;   first poppet valve means subject to actuation by the abutment to curtail flow of fluid from the brake pipe to the quick action chamber upon the occurrence of an emergency rate of reduction in brake pipe pressure;   second poppet valve means subject to actuation by the abutment to curtail flow of fluid from the quick action chamber to atmosphere upon occurrence of an emergency rate of reduction in brake pipe pressure; and   third poppet valve means subject to actuation by the abutment to direct flow of fluid from the quick action chamber to actuate the vent valve means upon the occurrence of an emergency rate of reduction in brake pipe pressure. .Iaddend..Iadd. 90. A fluid brake control system according to claim 44 wherein the proportioning valve means comprises stepped piston means for sensing when a force on one face of the piston means due to brake cylinder pressure substantially equals a total force on an opposite and larger face of the piston means due to pressure from the first reservoir and thereafter for lapping the charging of the brake cylinder. .Iaddend..Iadd. 91. A fluid brake control system according to claim 90, wherein said stepped piston has a first, smaller face and a second, larger face oriented opposite to said first face, said first face being exposed to brake cylinder pressure and said second face being exposed to pressure of fluid flowing from the first reservoir. .Iaddend..Iadd. 92. A fluid brake control system according to claim 90 wherein the stepped piston means has an associated chamber subject to brake pipe pressure for preventing operation of the proportioning valve means during a service brake application. .Iaddend..Iadd. 93. A fluid brake control system according to claim 91, wherein said piston comprises a third face oriented opposite to said second face, said third face being exposed to brake pipe pressure, whereby said piston is biased to permit charging of the brake cylinder only from the first reservoir during a service brake application. .Iaddend..Iadd. 94. A fluid brake control system according to claim 91, wherein said first smaller face is sized relative to said second, larger face so that during an emergency application the piston moves to close both the first and second valves when the pressure in the brake cylinder is a fixed percentage above service braking pressure. .Iaddend..Iadd. 95. A fluid brake control system according to claim 94, wherein the fixed percentage of service braking pressure is approximately 120%. .Iaddend.