Air Brake System Malfunctions and Emergencies
Air brake emergencies usually involve loss of air pressure or a component failure. If pressure drops, the low pressure warning activates, and if pressure continues to fall, the spring brakes apply automatically. Your job is to recognize the warning, stop safely, and diagnose the problem.
Common causes of pressure loss include leaks in lines, hoses, or fittings, a failed compressor, or a stuck valve. If you hear a leak, stop and inspect. If the compressor is not building pressure, the vehicle should not be driven until repaired.
If a brake chamber or diaphragm fails, you may hear a leak and notice reduced braking. In a dual system, one circuit may still function, but you should not rely on it for normal driving. Pull over and stop.
A spring brake may fail to release if air pressure is insufficient. Do not try to cage or bypass a spring brake unless trained and equipped; it is dangerous. If the parking brake will not release, the vehicle should be inspected.
Traps in questions include thinking you can drive normally with a known leak, or believing the spring brakes will not apply if you keep the engine running. They will apply when pressure is lost. Another trap is confusing a warning light with a minor issue; it is a signal to stop.
Rule of thumb: warning light means stop safely, find the leak, and do not continue until the system holds pressure.