Slack adjusters (also called brake adjusters or just "slacks") regulate the distance that the air brake has to travel to apply friction to the wheel. This distance increases as use of the brake causes the friction material of the brake shoe to wear away. When the brake is used, the operating rod pushes out on the slack adjuster which then turns the S-cam. The S-cam pushes the brake shoes apart which applies friction and slows the vehicle. Without this function, the air brake can become out of adjustment and the vehicle may lose its capability to slow or stop consistently. Haldex, EKU, Meritor, Stemco.