If your truck or bus has dual parking control valves, you can use pressure from a separate tank to:

Question

Accepted Answer

A: Release the spring emergency/parking brakes to move a short distance.. Parking and emergency brakes are currently spring-loaded mechanical brakes. The springs are held back by air pressure, so they are not activated when air pressure is present. When air pressure is removed the springs move into place to activate the mechanical brakes. Pressure from a separate tank can be used to hold the springs back to move a short distance.

If your truck or bus has dual parking control valves, you can use pressure from a separate tank to:

Idaho CDL Air Brakes Test Info