Drum brakes are used on all air foundation brakes systems. When the camshaft (20) turns, the brake linings are forced outward in order to contract the brake drum. This provides the desired clamping action for braking.

Applying the brake pedal sends air pressure to the air brake chamber at each wheel. The air brake chamber pushrod moves the air brake automatic slack adjuster (8), rotating the camshaft (20) and expanding the brake shoes and the brake linings (15) in order to contact the brake drum. Releasing the brake pedal allows air pressure to be exhausted from the air brake chambers. The pushrod return spring and the brake shoe return springs (17) pull the brake shoes and the brake linings (15) away from the brake drum.

The camshaft (20) has an S-shaped cam on one end and splines on the other end. The camshaft splines fit together with the air brake automatic slack adjuster (8). The cam pushes against the brake shoe rollers (1) in order to expand the brake shoes and the brake linings (15). The sealed bracket (10) with bushings (11) houses and supports the camshaft (20). The camshaft (20) is lubricated through an external fitting in the bracket.

The air brake automatic slack adjuster (8) fits together with the camshaft splines and acts as a lever in order to turn the camshaft (20). The air brake automatic slack adjuster (8) contains a worm gear in order to adjust the angle in relation to the camshaft (20). This provides a way of adjusting the brakes in order to compensate for normal brake lining wear. The air brake automatic slack adjuster (8) is standard on all models.

The length of the air brake chamber pushrod stroke is set at the time the air brake automatic slack adjuster (8) is installed. The length of the stroke is related to the clearance between the brake linings and the brake drum.

When the brake linings wear, the clearance increases. The increased clearance makes the air brake chamber pushrod move a greater distance in order to apply the brakes.

If the stroke length exceeds the design limit, the air brake automatic slack adjuster (8) makes an adjustment during the return stroke of the pushrod. The adjustment corrects for brake lining wear and automatically resets the stroke to the correct length. The cycle then repeats.

The air brake chamber clevis connects the end of the air brake service chamber pushrod to the air brake automatic slack adjuster (8). The air brake chamber clevis pin(s) fits through the air brake chamber clevis and the air brake automatic slack adjuster (8) end in order to allow the needed rotational movement.

Front air brake chambers use compressed air in order to create mechanical motion. As the air pressure enters the sealed side of the front air brake chamber diaphragm the pushrod is moved outward. The diaphragm enables the air pressure to cause the mechanical movement required and seals the front air brake chamber in order to keep air from leaking out.

Rear air brake chambers have two separate air chambers mounted one behind the other. The front section of the assembly is the service brake chamber that is operated by the brake pedal. The rear section of the assembly is the spring brake chamber and has a large mechanical spring that forces the pushrod outward when the air pressure is exhausted. The spring brake chamber provides fail-safe braking in emergencies or parking situations.

Releasing the parking brake involves sending air pressure to the spring brake chamber. The air pressure compresses the diaphragm and the compression spring in the spring brake chamber to release the brakes. When the brake pedal is applied, air enters the service brake chamber in order to apply the brakes. The compression springs automatically begins to apply the brakes if the air pressure drops below 655 kPa (95 psi). The brakes are fully applied when the air pressure drops to 379 kPa (55 psi).