Brake Failure Warning Signs and Emergency Procedures
This domain tests recognition of air brake failure and the correct emergency response. Exam items focus on the low-pressure warning, spring brake application, loss of one circuit, and brake fade or fire.
Warning signs
- Low-pressure warning light and buzzer - required to activate before pressure falls below 60 psi.
- Supply gauge falling steadily with the engine running; compressor not building pressure.
- Spring brakes applying on their own (20–45 psi), usually with a noticeable drag or stopping force.
- Audible hiss of escaping air; soapy bubbles at a fitting.
- Pedal that sinks, feels spongy, or goes to the floor.
- Brake fade after heavy use; pulling to one side; smoke or burning smell.
Emergency actions
If the warning comes on while driving: stop as soon as it is safe, park, and do not continue. The system is losing pressure and spring brakes will soon apply. If service brakes fail entirely, apply the parking/spring brakes gradually rather than yanking the knob, downshift to use engine braking, look for an escape ramp, and if nothing else works, rub against a guardrail or curb to shed speed. If one circuit of a dual system fails, the other should still provide braking - but only enough to stop safely, not to continue the trip. For smoke or fire, stop, shut off the engine, and get out.
Worked example
At 55 mph the low-pressure buzzer sounds and the supply gauge reads 55 psi and falling. The driver should not try to reach the next exit: pressure will reach the 20–45 psi spring-brake range and the brakes will lock on. The correct response is to signal, move to the shoulder, stop, and shut down.
Traps
Pumping the brakes to “build pressure” when the compressor has failed; using the parking brake suddenly at highway speed; and treating a low-pressure warning as a nuisance rather than a stop-now signal.