In a vehicle with dual parking control valves, a separate air tank can be used:

Question

In a vehicle with dual parking control valves, there is a separate air tank that can be used to temporarily release the spring brakes if they have been activated due to low air pressure. Pushing in the proper control will release the spring brakes for a short period of time, allowing the driver to move the vehicle in an emergency.

Accepted Answer

To temporarily release the spring brakes.

Answer

To make the air brakes work more effectively.

Answer

To charge the brakes on the rear trailer.