A fluid pressure control system engages the power take-off of a tractor-trailer combination vehicle equipped with air brakes while simultaneously applying the parking brakes of both the tractor and the trailer. However, when the vehicle is to be moved and the power take-off disengaged, the vehicle operator must, in addition to moving the power take-off control to the position disengaging the power take-off, must also operate the parking control valves which release the parking brakes on both the tractor and the trailer. Accordingly, the parking brakes cannot be inadvertently released by merely disengaging the power take-off control, although engagement of the power take-off control automatically sets or applies the parking brakes on both the tractor and the trailer.
A control valve for a fluid pressure braking system includes a housing which receives a pair of identical plungers and corresponding springs acting on the plunger which activate automatically at different trip pressures. The differential trip pressures are attained by providing different valve seating areas on the housing and spring cavities of different depths.
A brake system for a tractor-trailer vehicle includes a service brake system and a park brake system. The service and park brake systems are used to brake both a tractor and a trailer towed by the tractor. A trailer hand control valve is actuated by a vehicle operator to brake the trailer. The park brake system includes a park brake control valve that receives input commands from the vehicle operator to apply and release park brakes on the tractor and the trailer. An interlock valve assembly fluidly connects the park brake control valve to primary and secondary air supplies. The trailer hand control valve is also fluidly connected to the interlock valve assembly. The interlock valve assembly includes an outlet port that supplies an air signal to the trailer from one of the primary air source, secondary air source, or trailer hand control valve via a single connection line.
