Figure 12 illustrates the common S-cam brake assembly used on truck and trailer axles. Front brake assemblies have the brake chamber and slack adjuster mounted on the backing plate because the steering action of the front axle would otherwise interfere.

The diagram shows the brakes in the applied position. The S-cam is rotated so the high points have acted against the cam rollers and forced the brake shoes against the drum.

When the brakes are released, the brake cam shaft returns the brake cam to the normal position. The cam rollers roll down into the crook of the S-cam as the brake shoe return spring pulls the shoes away from the drum.

Brake lining material is attached to the face of the shoes. Lining material is selected according to the type of service the brakes are subjected to. Linings must give consistent braking output with minimum fade at high temperatures.

Brake shoes generate heat through friction with the brake drum surface. Drum thickness determines the amount of heat that can be absorbed and dissipated to the atmosphere. Thin or distorted drums, weak return springs, improper linings, poor adjustment, or grease or dirt on the lining, will all result in erratic, unpredictable and potentially dangerous brake performance.