When a vehicle needs to brake, air enters the air brake chamber from the air inlet and a flexible rubber disc called a diaphragm deforms under air pressure, causing the push rod to move. This movement spreads the braking force over the brake shoes, which are pressed onto the drum and slows or stops the wheel.
The braking system includes a master cylinder, which converts the pressure on the pedal into hydraulic pressure. It also has a piston in each end of the cylinder that forces one of the long, curved brake shoes against the drum. The leading shoe (closest to the front of the vehicle) is pushed toward the drum and the trailing shoe (adjacent to the rear of the vehicle) is pushed away from the drum.
Brake shoes are a major component of the braking system, and should be inspected regularly to make sure they're not too worn or broken. A worn brake shoe can cause the car to pull when you press down on the pedal.
Another important part of the braking system is the backplate, which supports the rotor and absorbs the braking force. It must be made of heat-resistant and wear-resistant material.
There are two main types of air brake chambers - service and spring brake. A service brake chamber has a diaphragm, a push rod and a return spring. When a vehicle brakes, compressed air fills the chamber, causing the diaphragm to move and push out the push rod. When the air pressure is released, the pushrod returns to its original position.