CDL Air Brakes TestAir Brake System PartsMedium
A driver notices that after building full air pressure, the air compressor continues to run, but no air is being pumped into the air tanks. What is the most likely reason for this observation?
- AThere is a significant leak in the service brake lines.
- BThe air compressor governor is stuck in the 'cut-in' position.
- CThe safety valve on the air tank has opened.
- DThe air compressor's unloader valves are open.
Show answer & explanationAnswer & explanation
Correct answer: D. The air compressor's unloader valves are open.
When the air system reaches full pressure, the compressor governor signals the unloader valves to open. This allows the compressor to run without pumping air, preventing over-pressurization and reducing wear.
Why the other options are wrong
- A. A leak in the service brake lines would cause the compressor to cycle more frequently to maintain pressure, not to run continuously without pumping air.
- B. If the governor were stuck in 'cut-in', the compressor would continue to pump air, leading to over-pressurization.
- C. The safety valve opening indicates over-pressurization, which would occur if the unloader valves failed to open, not if the compressor ran without pumping air.
Air Compressor Unloader Valves
These valves allow the air compressor to run without pumping air into the air tanks once the system reaches its maximum pressure, preventing over-pressurization.
- Controlled by the air compressor governor.
- Open when cut-out pressure is reached.
- Close when cut-in pressure is reached.
- Reduce wear on the compressor and prevent system over-pressurization.
Memory trick: Unload the pressure, let the compressor rest, but keep it ready for the next test.