Air drum brakes use air to rotate a shaft, called the s-cam, which causes the brake shoes to press against the brake drum to stop a vehicle. The air brake system is complemented with a back up arrangement of parking and emergency brakes powered with springs.
Checking and adjusting air drum brakes is a simple procedure that is extremely important for road safety. With the air brakes full of air pressure and the wheels chocked, or secured, release the brakes. Allow the push rod to exit the air chamber and mark that position with chalk. Pull the rod completely out of the chamber and note the distance between the chalk mark and the location of the chamber.
If the distance is more than 3/4-inch, turn the push rod until resistance is felt of the shoes pressing against the drums. Back the rod out by 1/4 to 1/2 turn to properly adjust the system.
