CDL Air Brakes TestAir Brake System PartsMedium
A technician is diagnosing an issue with a commercial vehicle's air brake system where the air compressor runs continuously, but the air pressure never exceeds 100 psi. What is the most likely cause?
- AA stuck-open safety valve on an air tank.
- BA malfunctioning air compressor governor (cut-out setting).
- CExcessive air usage from auxiliary systems.
- DA faulty low air pressure warning switch.
Show answer & explanationAnswer & explanation
Correct answer: A. A stuck-open safety valve on an air tank.
If the safety valve on an air tank is stuck open, it will continuously release air, preventing the system from building up to its maximum operating pressure (typically 125 psi) even if the compressor is working.
Why the other options are wrong
- B. A malfunctioning governor (cut-out) would either not cut out at all (leading to over-pressurization) or cut out too early (but the question states pressure never exceeds 100 psi, implying a leak, not a premature cut-out).
- C. While excessive auxiliary usage could lower pressure, a continuous run with a hard limit at 100 psi points more directly to a constant, large leak like a stuck safety valve.
- D. A faulty low air pressure warning switch would affect the warning system, not the maximum pressure the system can build.
Air Tank Safety Valve
A pressure relief device on air tanks designed to protect the air brake system from over-pressurization.
- Opens automatically if pressure exceeds 150 psi.
- Prevents damage to components and potential tank rupture.
- Should 'pop off' if governor fails to cut out compressor.
Memory trick: Safety valve is the system's pressure guardian.