Air Brake System Malfunctions and Warning Signals
Air Brake System Malfunctions and Warning Signals
This domain tests what the gauges, lights, buzzers, and vehicle behavior tell you, and what to do before a small fault becomes a brake failure. Questions often use a pressure drop, a warning that stays on, or a compressor that will not keep up.
Core rules
- The low-air warning light and buzzer must activate before 60 psi. A steady warning means stop safely and find the cause; do not continue a trip.
- Spring brakes apply automatically at roughly 20–45 psi. They are not a service brake and can lock wheels if they apply at speed.
- Common causes: compressor failure, governor failure, leaking lines or chambers, stuck drain valves, frozen air lines, oil or water in the system, and brakes out of adjustment.
- A compressor that cannot build from 85 to 100 psi in 45 seconds, or cannot keep up with demand, is defective.
- Draining reservoirs daily removes water and oil. Alcohol evaporators or air dryers help in cold weather, but they do not replace draining.
- If a brake drags or smells hot, stop and check; continued driving can cause fire or fade.
Worked example
A driver sees the gauge fall from 120 to 55 psi on a level road with no brake application. The low-pressure warning sounds at 60 psi, and the spring brakes apply near 30 psi. The correct action is to pull off the road while the service brakes still have pressure, not to pump the pedal or drive to the next exit. The rapid drop points to a major leak or compressor failure.
Traps
Do not confuse the warning pressure (60 psi) with spring-brake application pressure (20–45 psi). Do not pump air brakes when pressure is falling; pumping uses more air. Do not assume a warning that resets means the fault is gone.