Abstract: A valve assembly for use in conjunction with spring applied, air release parking brakes of the character found in heavy-duty truck-trailers which, in a single valve housing provides system pressure protection, prevents compounding application of the brakes and performs the parking brake release function in an expedicious and highly reliable manner.

Abstract: A brake system for a vehicle having two sets of brakes, a front set and a rear set of brakes and including a source of fluid pressure, a driver controlled valve, in which the sets of brakes respond to fluid pressure from said source via said driver controlled valve, including a secondary valve having at an input port a connection to the driver controlled valve and its output port coupled to the rear brakes, and in which the secondary valve means produces an increase in pressure at the output port above the pressure of the input port, in which the secondary valve including time delay means for reducing excess pressure at the outlet port to reduce the pressure at the outlet port to substantially the pressure at the inlet port after the application of pressure to the inlet port by operation of the driver controlled valve.