Air Brake System Malfunctions and Emergency Procedures
Emergency questions reward one response pattern: recognise the symptom, stop safely, and do not move the vehicle until the fault is corrected.
Recognising the failure
Low air pressure is tested most. A warning light and buzzer at 60 psi, spring brakes setting between 20 and 45 psi, and a rapid gauge drop with the engine off all point the same way: stop as soon as it is safe, park, set the parking brake, and do not move until the leak is found.
If pressure is lost while moving
Downshift progressively, keep the engine running, and brake with whatever remains. Spring brakes apply fully and cannot be modulated, so treat them as a last resort and expect a hard, sudden stop. Never shut the engine off to "save air" - you lose the compressor and power steering assist.
Tractor protection and breakaway
The tractor protection valve closes the tractor's air supply to the trailer around 20–45 psi, preserving tractor braking. If a trailer breaks away, the emergency line separates and the trailer's spring brakes set automatically.
Other emergencies
- Runaway on a grade: use the escape ramp; do not jump.
- Brake fire or smoke: stop and let drums cool; do not spray water on hot drums.
- Compressor failure: pressure will not build; the unit must be repaired or towed.
- To move a vehicle with disabled spring brakes, cage the chambers with the release bolt per the manufacturer's procedure.
Worked example
A driver sees the low-pressure light at 55 psi on a downgrade. Reasoning: the warning is real and spring brakes will set soon, so the driver downshifts, moves to the shoulder, and stops before pressure reaches the 20–45 psi spring-brake range rather than being surprised by an automatic application in a curve.
Traps
Do not choose "pump the brakes to build pressure", and do not describe spring brakes as a smooth service brake.