To maintain your semi truck in sound working order, repairs and service need to be able to perform a host of tasks to keep a steady flow of air compressed and directed to all the necessary systems. As a truck driver presses on the brake pedal, the air is moved through the lines into the brake chamber. The compressed air is moved against a push rod that then pushes against the slack adjuster, which turns the S-Cam allowing the brake linings to reach the brake drum, which creates friction to slow the truck down. Every part of your needs to be in working order for this process to work and your brakes to be effective.