CDL Air Brakes TestAir Brake System PartsHard
A technician is performing a detailed inspection of a commercial vehicle's air brake system. They observe that the air compressor's unloader valves are not seating properly, causing a constant, faint hiss from the compressor head when the system is at cut-out pressure. What is the primary consequence of this condition?
- AThe safety valve will frequently open.
- BPremature wear of the air compressor.
- CExcessive moisture accumulation in the air tanks.
- DAn inability for the air compressor to build pressure.
Show answer & explanationAnswer & explanation
Correct answer: B. Premature wear of the air compressor.
The unloader valves release pressure from the discharge line when the governor signals the compressor to 'cut out'. If they don't seat properly, the compressor works against residual pressure in the discharge line and may continue to pump a small amount of air, even when it's supposed to be 'unloaded' and idling. This continuous, low-level work leads to premature wear and overheating of the compressor, as it's not truly 'resting' when it should be.
Why the other options are wrong
- A. The safety valve opens if pressure exceeds its limit (e.g., 150 psi). Leaking unloader valves prevent the system from truly reaching cut-out (125 psi) and would not typically cause overpressure that triggers the safety valve.
- C. Moisture accumulation is related to the air dryer's function, not directly to leaking unloader valves.
- D. The compressor can build pressure, but the issue is its inability to properly 'rest' or unload once cut-out is reached.
Air Compressor Unloader Valve Leak
A condition where the unloader valves in the air compressor do not properly seal, preventing the compressor from fully 'unloading' and resting at cut-out pressure.
- Causes compressor to work continuously against residual pressure.
- Leads to overheating and accelerated wear of compressor.
- Often indicated by a faint hiss at the compressor head when system is at full pressure.
Memory trick: Unloader Leaks, Compressor Creaks, Life Decreases.