Air Brake System Parts and How They Work
Air brakes use compressed air to make a heavy vehicle stop. They are a safe and effective way to stop large trucks and buses because air is always available and the system stores a reserve of pressure, so a small leak will not immediately stop the brakes from working. But an air brake system has many parts and takes more skill to inspect and use than a hydraulic system. This chapter explains the major components: how air is made and stored, how it is turned into braking force at the wheels, the spring brakes that stop you if air is lost, and the dual-system design that gives heavy vehicles a backup. Understanding each part helps you recognize problems during inspection and drive safely.
Making and Storing Air: Compressor, Governor, and Tanks
The air compressor pumps air into the storage tanks (reservoirs) where compressed air is kept until it is needed for braking. The compressor is driven by the engine through gears or a belt and may be air-cooled or connected to the engine cooling system. The governor controls when the compressor pumps air into the tanks. When tank pressure reaches the cutout level, the governor stops pumping; when pressure falls to the cut-in level, it starts pumping again. Because compressed air always contains some water and oil, tanks have drain valves at the bottom to remove this buildup, which can otherwise freeze in cold weather and damage the brakes.
Turning Air Into Braking Force: Chambers, Slack Adjusters, and S-Cams
Pressing the brake pedal (also called the foot valve or treadle valve) sends air to the brake chambers at each wheel. Air pushes the pushrod out of the brake chamber, and the pushrod moves the slack adjuster, which rotates the S-cam. The S-cam forces the brake shoes apart and presses their linings against the inside of the brake drum, creating the friction that slows the wheel. The slack adjuster keeps the correct clearance between the linings and the drum as the linings wear. Pressing the pedal harder sends more air and creates more braking; releasing the pedal lets the air out and releases the brakes.
Spring Brakes and the Parking Brake
Trucks and buses must have emergency and parking brakes that work mechanically, because air pressure can eventually leak away. Spring brakes serve this purpose: powerful springs apply the brakes, and air pressure holds the springs back so the brakes stay released while driving. When air pressure is lost, the springs push the brakes on. The parking brakes are set with a separate control valve, usually a yellow diamond-shaped knob; pulling it out applies the spring brakes and pushing it in releases them. Because the parking brakes are held on by springs and not by air, they will stay applied even if air leaks out overnight.
Dual Air Systems and Safety Valves
Heavy vehicles use a dual air brake system with two separate air systems operated by a single set of controls. One system typically operates the rear brakes and the other the front brakes; if one system loses air, the other can still stop the vehicle. Before driving, you must let pressure build to at least 100 psi in both the primary and secondary systems. Several protective valves keep the system safe: a one-way check valve keeps stored air from flowing back out if the supply side leaks, and a safety relief valve protects the tanks from too much pressure, usually opening near 150 psi if the governor fails. In cold climates, an alcohol evaporator may add alcohol to reduce ice in the valves.
Last updated: July 2026