CDL Air Brakes TestAir Brake System PartsMedium
A technician is troubleshooting a commercial vehicle and finds that the air compressor is constantly running, but the air pressure in the tanks never reaches the cut-out pressure. The safety valve on the air tank is not releasing air. What is the most likely cause?
- AA major leak in the air brake system.
- BA stuck-open air compressor governor.
- CA faulty low air pressure warning switch.
- DA clogged air dryer desiccant cartridge.
Show answer & explanationAnswer & explanation
Correct answer: A. A major leak in the air brake system.
If the compressor runs continuously but pressure doesn't build to cut-out, and the safety valve isn't releasing (indicating overpressure isn't the issue), the most probable cause is a significant air leak somewhere in the system that prevents pressure from accumulating. The compressor is working, but the air is escaping.
Why the other options are wrong
- B. A stuck-open governor would prevent the compressor from building pressure at all, or cause it to build pressure too slowly, but wouldn't explain continuous running without reaching cut-out if there's no leak.
- C. A faulty low air pressure warning switch would affect the warning, not the ability of the system to build pressure.
- D. A clogged air dryer would restrict airflow, potentially causing slow build-up or compressor overheating, but a major leak is more likely to prevent pressure from ever reaching cut-out while the compressor runs constantly.
Continuous Compressor, No Cut-Out
A diagnostic symptom where the air compressor runs constantly but fails to reach the upper pressure limit (cut-out pressure) in the air tanks.
- Often indicates a significant air leak in the system.
- Could also be a severely worn compressor, but leak is more common.
- Safety valve not opening rules out overpressure.
Memory trick: Constant Pump, No Peak, Means Leak.