Air Brake Malfunctions and Emergency Procedures
Air Brake Malfunctions and Emergency Procedures
This domain tests what you should recognize as an air-brake failure and what you must do immediately. The exam favors safe, controlled responses over heroic attempts to keep driving.
Core rules
- A low-pressure warning means the system is losing its ability to apply service brakes. Move to a safe stop; do not continue to the next stop "to check it."
- If pressure keeps falling, spring brakes will set automatically in the 20-45 psi range. That is designed fail-safe behavior, but it can stop the vehicle in a bad location, so act before it happens.
- A rapid leak, broken line, or compressor failure can drain pressure quickly. Stop, secure the vehicle, and do not drive until repaired.
- If service brakes fail on a downgrade, downshift progressively, use engine braking, and look for an escape ramp or safe runoff. Use the parking/spring brake only as a last resort, because it can lock the rear wheels.
- Do not pump the brake pedal repeatedly when pressure is low; it uses air faster and can bring spring-brake application sooner.
- After any repair or air-loss event, repeat the build-up, leak, warning, and spring-brake checks before moving.
Worked example
While driving, the low-pressure buzzer sounds and the gauge falls from 110 to 55 psi. The driver should signal, move to the shoulder, and stop while steering and some braking remain. Waiting for the spring brakes to set at 35 psi might leave the vehicle stopped in a travel lane or on a curve. Once stopped, the driver should not continue until the leak is found and corrected.
Traps
Do not ignore a warning because the pedal still feels firm. Do not pump the brakes to "build pressure." Do not assume the parking brake can substitute for service brakes at highway speed. Do not drive after a low-pressure event without a full re-test.