When you press the brake pedal, compressed air fills an air brake chamber, causing a rubber disc called a diaphragm to move and push out a metal rod, known as a pushrod. When you release the brake pedal, the pressure is released and the pushrod returns to its original position.

Service/Spring Brake Chambers (SBC)
There are two main types of air brake chambers used on commercial vehicles: service and spring brake chambers. Each type uses a different mechanism to apply the brakes.
A service brake chamber contains a flexible diaphragm, a push rod and a return spring. When you apply the brakes, the force of the pushrod transfers to the spring, which applies the brake shoes or pads.
Adjustment Limits for Brake Chambers
As the air brakes wear, the linkage that connects the pushrod to the brake assembly must be re-adjusted at regular intervals. This re-adjustment moves the pushrod farther from the brake chamber, and it also allows the brake shoes or pads to contact the drum or disc.
Brake Rotor & Pads
The most common part that wears on an air disc system is the rotor and the pads. Due to the smaller size of the rotor and the air disc pads, they are exposed to more heat than a drum caliper setup. This can lead to premature wear on the rotor and pad.