Air Brake System Malfunctions and Warning Signals
The Malfunctions and Warning Signals domain tests whether you can read the gauges and warning devices and decide what a failure means. The exam favors questions about what happens at specific pressures and which device signals which problem.
Pressure thresholds to memorize
- Below 60 psi: the low-pressure warning light and buzzer activate. This is a stop-and-fix signal, not a suggestion.
- 20 to 45 psi: spring brakes apply automatically as air escapes.
- About 150 psi: the safety valve opens to protect the system; a governor failure is a common cause.
Common failures
- Compressor or governor failure: pressure builds slowly or not at all. In a dual system, pressure should build from 85 to 100 psi within 45 seconds at fast idle.
- Supply leak: pressure falls even when you are not braking.
- Service-side leak: pressure falls only while the brake is applied.
- Tractor protection valve: closes and pops the trailer air supply knob out when pressure drops, sealing off a leaking trailer.
- ABS light: ABS is disabled, but normal air brakes still work; the vehicle can be driven, unlike with a low-air condition.
- Brake fade: overheated linings lose grip; the pedal may feel normal while stopping power drops.
Worked example
A driver cruising at 100 psi sees the gauge slide to 70 psi with no brake application and hears nothing unusual. Because pressure is falling with the brakes released, this is a supply leak, not a service-brake leak. Waiting for the 60 psi warning is wrong; the driver should stop in a safe place immediately, because spring brakes will apply between 20 and 45 psi.
Traps that cost marks
- Treating the 60 psi warning as a low-fuel-style reminder.
- Confusing the pressure at which the warning sounds with the pressure at which the spring brakes apply.
- Assuming an ABS light means total brake failure.
- Diagnosing a leak as normal because the compressor keeps up.