How an air brake system works
An engine-driven compressor fills reservoirs (air tanks) to a regulated pressure set by the governor. When the driver presses the brake pedal, the foot valve sends air to the service brake chambers. Each chamber pushes a pushrod, which turns a slack adjuster and an S-cam or moves a disc caliper, forcing the friction material against the drum or rotor.
Spring brakes work the other way. A strong spring applies the brake, and air pressure holds it off. That is why spring brakes act as the parking brake and apply automatically if air pressure is lost.
Main components
- Compressor, governor and air dryer.
- Supply, primary and secondary reservoirs.
- Foot valve, relay valves and quick release valves.
- Service and spring brake chambers. See brake chamber.
- Slack adjusters. See slack adjuster.
- Drums and shoes, or rotors and pads.
- Tractor protection valve and gladhands for trailers.
Rules that apply
49 CFR 393.40 requires commercial motor vehicles to have service, parking and emergency brake systems. 49 CFR 393.47 sets brake chamber sizing, lining thickness and the pushrod stroke readjustment limits. Air brake defects such as leaks, out-of-adjustment brakes and worn linings are among the items checked in the annual inspection under Part 396.
Why it matters to repair shops
Air brake work is routine in heavy-duty shops: chamber replacement, slack adjuster service, lining replacement, leak repair and dryer cartridge changes. Anyone who inspects, maintains, repairs or services brakes on a motor carrier's vehicles must meet the brake inspector qualifications in 49 CFR 396.25. See brake inspector.
Related: MetaFleet inspections records DVIRs and inspections with photos; get a walkthrough.