Air brakes
Air brakes use compressed air to stop the vehicle. An engine-driven compressor pumps air into storage tanks. A governor stops the compressor at about 125 psi (cut-out) and starts it again around 100 psi (cut-in). A safety valve opens at about 150 psi if pressure gets too high.
There are three systems: service brakes for normal stopping, parking brakes, and emergency brakes. Spring brakes are held off by air pressure. If pressure falls to between 20 and 45 psi, the springs apply the brakes automatically. Before that, a low-pressure warning must come on before pressure drops below 60 psi. If it does, stop safely as soon as you can.
Air brakes have brake lag: air takes about half a second to reach the brakes, which adds about 32 feet of stopping distance at 55 mph. Water and oil collect in the tanks, so drain them daily if they don't drain automatically. In cold weather, water can freeze and block the brakes.
Learn the leakage test. With the engine off and brakes released, a single vehicle should lose less than 2 psi per minute (3 for a combination). With the pedal held down, less than 3 psi per minute (4 for a combination). Pressure should build from 85 to 100 psi within 45 seconds in a dual air system.
