CDL General Knowledge TestDriving SafelyHard
You are preparing to drive a commercial vehicle equipped with air brakes. During your pre-trip inspection, you notice that the air pressure buildup time from 85 psi to 100 psi takes 60 seconds. What does this indicate?
- AThere is a minor leak in the air brake system.
- BThe air tanks are too large for the compressor.
- CThe air compressor is not building pressure fast enough, indicating a problem.
- DThe air compressor is functioning normally.
Show answer & explanationAnswer & explanation
Correct answer: C. The air compressor is not building pressure fast enough, indicating a problem.
For a dual air system, the air pressure should build from 85 to 100 psi within 45 seconds. A 60-second build-up time indicates the compressor is not building pressure fast enough, suggesting a potential issue with the compressor or a significant leak.
Why the other options are wrong
- A. A minor leak would likely result in a slower drop, not necessarily a slow build-up, or would fail a pressure drop test.
- B. Tank size is generally matched to the compressor; a slow build-up points to a compressor or leak issue, not a sizing mismatch.
- D. Normal function requires pressure to build within 45 seconds, so 60 seconds is not normal.
Air Brake Build-Up Time
The time it takes for the air compressor to build pressure from 85 psi to 100 psi (or 50-90 psi for older systems), which should be no more than 45 seconds for a dual air system.
- Tests compressor efficiency and system integrity.
- Excessive time indicates a problem.
- Critical safety check for air brake systems.
Memory trick: Eighty-five to hundred, in forty-five, quick as a thunder.