Abstract: A relay valve for a fluid pressure braking system includes a relay piston and a load piston which is responsive to the pressure at the inlet port of the relay valve to exert a biasing force on the relay piston loading the latter against the inlet/exhaust valve. The biasing force is opposed by fluid pressure at the inlet port acting across the effective area of the inlet/exhaust valve. The biasing force is greater than the spring force holding the inlet/exhaust valve closed, so that the valve responds immediately to pressure signals transmitted through the control port of the relay valve.

Abstract: A dual circuit brake valve includes a plunger assembly carrying the primary valve seat, and the primary valve plunger within the primary piston. A pair of axially-spaced, circumferentially extending bearing members extend from the primary piston slidably engaging larger and smaller diameter portions of the bore of the housing to thereby guide the primary piston independently of other components within the housing. The secondary set of valve actuating members is actuated by a relay piston which is similarly guided in larger and smaller diameters of the bore independently of other components within the valve housing. The primary piston defines a pressure effective area due to the difference in the larger and smaller portions, so that inlet port pressure acting on the effective area yieldably urges the plunger toward the brake released position.

Abstract: A dual circuit brake valve for an articulated vehicle is responsive to a pressure signal provided by the system tractor protection valve to provide a full brake application in both the primary and secondary braking circuits actuating the front and rear wheel brakes of the vehicle respectively when the portions of the articulated vehicle are connected for movement together. A proportioning piston is responsive to the pressure signal to proportion braking pressure in the secondary when the towing portion of the vehicle is operated in the "bobtail" mode without the towed portion of the vehicle. The brake valve includes mechanism for mechanically actuating the secondary circuit valve to provide a full brake application under emergency conditions during operation of the vehicle in the bobtail mode.