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Push in on the parking brake switch in order to release the parking brake. The parking brake control module (PBCM) turns on the pump motor in order to supply fluid pressure to the actuator. There is a large spring inside of the actuator that applies the parking brake. The piston moves in the actuator when the fluid pressure overcomes the spring tension. This movement transfers to the parking brake through the parking brake cable. The pump shuts off and the PBCM monitors the system pressure when the parking brake is released. The PBCM turns on the pump in order to build pressure if the pressure drops too low.
Pulling out on the parking brake switch de-energizes the parking brake solenoid in order to dump the fluid into the reservoir. The spring in the actuator moves the piston and applies the parking brake through the movement of the parking brake cable as the pressure decreases.