Low Air Pressure Warning Devices
The Low Air Pressure Warning domain asks where the warning must trigger, how to test it, and what a driver must do when it fires. It is one of the most heavily weighted air brake topics.
The rule
- Every air-braked vehicle needs a low air pressure warning - a lamp, a buzzer, or both - that activates before pressure in either circuit falls below 60 psi.
- The warning is your last clear notice before the spring brakes apply automatically at roughly 20–45 psi. Between those two events the driver is expected to have already stopped.
How to test it
With the engine off and the key on, pump the pedal to bleed pressure down while watching the gauge. Note the pressure at which the warning first activates. Activation at 58 psi passes; activation at 50 psi fails.
If it activates in service
- Move off the roadway and stop in a safe place. Do not try to reach the terminal.
- Keep the engine running to preserve what pressure remains while you get clear of traffic.
- Secure the vehicle: parking brake, chocks, wheels turned into the curb.
- The vehicle stays out of service until the leak or the warning fault is repaired.
Worked example
A driver says "the light flickers on around 45 psi, but the brakes still worked, so I kept going." There are two errors: the warning is late, since the spec is before 60 psi, and continuing after a low-pressure warning is exactly what the exam expects you to reject. The correct answer is to stop safely and park it.
Traps
Confusing 60 psi (warning threshold) with 20–45 psi (automatic spring brake application); believing the warning is optional on trucks with a gauge; assuming the buzzer is only a parking aid; thinking the truck may be driven as long as pressure holds at 70 psi.