Normal Operating Checks and Warning Devices
Normal operating checks test what a driver monitors once moving: gauge pressure, warning devices, compressor recovery, and the first response to abnormal pressure. The exam separates normal governor cycling from a leak, a stuck compressor, or a saturated air dryer.
Gauge discipline
- Normal operating pressure is roughly 85-125 psi; the gauge should stay in that range.
- The low-pressure warning must activate before pressure falls below 60 psi; if it activates, stop safely and park.
- A safety valve near 150 psi is emergency protection, not a normal pressure regulator.
Compressor and dryer behavior
- After a brake application, the compressor should recover pressure to the normal range.
- Slow recovery, continuous low pressure, or frequent cut-in suggests a leak, worn compressor, or governor problem.
- The air dryer removes moisture; a saturated or frozen dryer can cause pressure and brake problems.
Brake application observation
- After a stop, check that pressure recovers and does not continue falling.
- Do not fan or pump the service brake; steady pressure gives better control and reveals leaks.
- If the gauge falls with the engine off and brakes released, perform the leakage test before driving.
Worked example
During a pre-trip, the low-pressure light comes on at 58 psi. That is a failure: the warning must come on before pressure drops below 60 psi. If the same light came on at 62 psi, it would meet the requirement. The driver should have the warning device repaired, not treat 58 psi as acceptable because the brakes still feel firm.
Traps that cost marks
- Thinking a warning at 55 psi is acceptable because the brakes still work.
- Ignoring a gauge that stays at 90 psi while the warning light is on.
- Using the safety valve as a normal way to limit pressure.
- Assuming slow compressor recovery is normal in cold weather.
- Record the defect and correct it before dispatch.