System Operation Air
The air-operated parking brake is used on the P92 chassis. The parking brake mounts on the rear of the transmission.
The air-operated parking brake assembly consists of the park brake valve, the pressure sensor, the park brake actuator, the drum and the shoe and lining assemblies.
The parking brake is spring applied but is controlled by air pressure. When the parking brake is applied, air pressure is released. This allows the spring to apply brake force. When the parking brake is released, pressure will hold it in that position, preventing the spring from applying the brake. The parking brake cannot be released unless there is a minimum of 414 kPa (60 psi) in system #1 of the brake system.
The parking brake is not designed for use in place of the service brakes. It should be applied only after the vehicle is brought to a complete stop, except in an emergency.
System Operation Hydraulic Auto Apply
The electric/auto parking brake system is used on the P72 chassis. This system uses a propeller shaft-mounted parking brake.
The parking brake is not designed for use in place of the service brakes. It should be applied only after the vehicle is brought to a complete stop, except in an emergency.
Move the shift lever on the steering column from the P (Park) position in order to release the electric/auto parking brake. Movement is transferred from the transmission shift linkage to the cam-actuated switch. The switch closes, allowing current to flow to the solenoid valve. The solenoid valve closes and holds system pressure. The cam-actuated switch also supplied current to the pressure maintenance switch. The pressure maintenance switch closes at pressures below 8 300 kPa (1200 psi), turning on the pump motor to supply fluid pressure to the actuator. The actuator has a large spring inside it that applies the parking brake. Fluid pressure overcomes spring tension and moves the piston in the actuator. This movement is transferred to the parking brake through the parking brake cable. When the fluid pressure reaches approximately 11 000 kPa (1600 psi), the pressure maintenance switch opens and the pump shuts off. The pressure maintenance switch opens and closes depending on the system pressure. If the pressure drops too low, the pressure maintenance switch turns the pump back on to build system pressure back up to its working range.
Putting the shift lever back into P (Park) or turning the ignition OFF de-energizes the parking brake solenoid to dump the fluid back into the reservoir. As the pressure decreases, the spring in the actuator moves the piston and applies the parking brake through movement of the parking brake cable.
This vehicle includes a feature that allows the parking brake to be applied and released manually when the shift lever is placed in N (Neutral). To release the parking brake, push in on the parking brake switch. Pulling out on the parking brake switch applies the parking brake. This is in addition to the electric/auto system.
