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Applies the parking brakes of the air brake equipped vehicle indefinitely – if needed of course.

The parking brakes are held in the “ON” position by the mechanical force of the springs; these spring parking brakes are held in the “OFF” position by system air pressure.

A powerful coil spring exerts force on the service brake components (shoes, linings, and drums).

Knowledge Test Question: If the service brakes are out of adjustment, so are the parking brakes owing to the fact that both use the same foundation braking components.

Springs brakes are referred to as "piggy back", owing to the fact that the spring brake chamber is bolted directly to the service brake chamber forming one unit.