Vehicle brake system and valve

A vehicle brake system comprising, a prime mover sub-system, at least one tandem trailer sub-system, a source of pressurized fluid, each said sub-system having a plurality of spring brakes, a distribution valve and a relay valve, each said spring brake having a spring actuator, a hold-back chamber and service chamber, each said distribution valve connecting said source to the hold-back chamber of each said spring brake in its respective said sub-system so as to thereby render the spring actuator of each said spring brake inoperative, each said relay valve being operable to connect said source to the service chamber of each said spring brake in its respective said sub-system so as to thereby operate each said spring brake, a control valve connected to both said relay valves and being operable to cause operation of each said relay valve and a control system which is operative to apply pressurized fluid to said control valve and thereby cause operation of that valve so that it exposes each said relay valve to pressurized fluid and thereby causes operation of said relay valves, whereby the brakes of each said sub-system are operated as a consequence of operation of said control valve.