First, the air compressor pumps air into the trucks air storage tanks, or reservoirs. That air is then pumped through the air brake lines where it eventually reaches the air brake. On most “S-Cam” brake systems, when you push the brake pedal on the truck, air pressure is going to push a rod out this will move the slack adjuster. The slack adjuster is used to essentially calibrate the brake system and ensure that the internal spring mechanism is working appropriately.
Air then flows through the nozzle to the air brake chamber, which then causes the spring to move the “S-cam.” The “S-cam” forces the brake shoe lining away from one another and presses them inside the brake drum. This creates the friction and pressure needed to slow the wheel down. Similarly, when you release your foot from depressing the brake pedal, the “S-cam” rotates back, causing the spring to pull those same brake shoes away from the brake drum eliminating the friction and slow-down effect.
The brake shoes wear over time especially with added pressure from depressing the brake pedal too often or too heavily. It is important that the part shoes are measured periodically. The minimum thickness required by CVIP regulations in Alberta is 8mm. It is always important to refer to the Commercial Vehicle Inception Manual if you are unsure of anything.