In order to park the vehicle a driver applies the spring brakes by pulling the valve(s) / button on the dash out. (Trailer brakes, red button; Tractor brakes, yellow button.) This has the effect of extinguishing the air from the brake chamber, releasing the spring inside.
This forces the pushrod out, which in turn pushes a slack adjuster, turning the camshaft, twisting the S-Cam and forcing the brake linings against the brake drum. Brakes are “set,” “applied” or “engaged” when the driver pulls one or both dash valves out.


The spring within the brake chamber has approximately 2500 lbs of pressure behind it, so never attempt to take a brake chamber apart!