CDL Air Brakes TestAir Brake System PartsMedium
A technician is troubleshooting a commercial vehicle's air brake system. They observe that even after the air compressor has built up to its maximum pressure, there is a continuous, loud hiss of air coming from a specific valve on one of the air tanks. What does this indicate?
- AThe brake pedal (foot) valve is leaking.
- BThe alcohol evaporator is over-pressurized.
- CThe safety valve on the air tank has opened.
- DThe air compressor governor is set too high.
Show answer & explanationAnswer & explanation
Correct answer: C. The safety valve on the air tank has opened.
A continuous, loud hiss of air from a valve on an air tank after maximum pressure is reached strongly suggests that the safety valve has opened. This valve is designed to release excess pressure to prevent tank rupture.
Why the other options are wrong
- A. A leaking brake pedal valve would typically cause air loss during brake application or from the pedal area, not a continuous hiss from an air tank.
- B. The alcohol evaporator prevents freezing, not typically a source of overpressure hissing from the tank itself.
- D. A governor set too high would lead to higher pressure, but the safety valve opening is the direct result of overpressure.
Safety Valve
A pressure relief device on each air storage tank designed to protect the tank and system from dangerously high air pressures.
- Opens automatically if pressure exceeds 150 psi (typically).
- Prevents tank rupture.
- Releases excess air with a loud hiss.
Memory trick: Safety Saves, Sounding an air-pressure alarm.