The slack adjuster is the part of an air-brake system that is used to adjust the brakes when needed. The slack adjuster is located on the air canister on the axle housing near the wheel. On vehicles with drum brakes, it is between the pushrod and the S-cam. On vehicles with disc brakes, it is between the pushrod and the power screw.
Air-brake systems use compressed air to transmit pressure from the brake pedal to the brake pads. They are typically used in larger vehicles such as trucks and buses. All air-brake systems run the risk of moving out of adjustment over time. Out-of-adjustment brakes are the most reported problem from roadside vehicle safety inspectors. Out-of-adjustment brakes constitute a safety hazard, as vehicles with excessive brake slack can be difficult to stop.
All vehicles manufactured since 1994 have automatic slack adjusters that adjust themselves during full brake applications. Automatic adjusters should only require manual adjustment during installation. If a brake system with automatic slack adjusters requires periodic adjustments, it is an indication that the automatic slack adjusters are defective and should be repaired. All brake systems, whether they have automatic or manual slack adjusters, should receive regularly scheduled maintenance checkups.