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.

A pneumatically operated brake booster comprises a hydraulic cylinder disposed within a hydraulic braking circuit connecting a master cylinder to wheel brake cylinders to increase the braking pressure trapped within the wheel brake cylinders and a servomotor for pneumatically activating the hydraulic cylinder in response to hydraulic pressure from the master cylinder. The servomotor has a spring loaded diaphgragm piston forming a negative pressure chamber and a variable pressure chamber, the former being in connection to a source of negative pressure and the latter being connected to the atmospheric pressure in response to master cylinder pressure applied to the hydraulic cylinder. The brake booster includes a control valve which is provided to selectively connect the variable pressure chamber of the servomotor to such a positive pressure source as exhaust pressure to obtain additional pressure in an emergent braking operation of the vehicle.

An electro-pneumatic braking system includes a tandem brake actuator including a service brake actuating section and an emergency/parking spring actuated section. Hold-off pressure is communicated to the emergency/parking section through a three-way solenoid valve from a pressure reservoir, and communication to the service actuator is controlled by an electro-pneumatic modulator. Service pressure is taken both from the branch of the conduit communicating with the air reservoir, and from the hold-off pressure in the emergency/parking reservoir. Accordingly, only a single line communicates fluid pressure from the reservoir to the brake, and service brake response time is improved because the hold-off pressure and the emergency/parking section of the actuator is used as a small reservoir immediately adjacent the service actuator.