CDL Combination Vehicles TestCombination Vehicle Air BrakesEasy
During a pre-trip inspection of a combination vehicle, you are checking the air brake system. You notice that the air compressor builds pressure from 85 psi to 125 psi in 35 seconds. The manufacturer's specification states that the compressor should build pressure from 85 psi to 125 psi in no more than 45 seconds with the engine at operating RPM. What should you conclude about this air compressor?
- AThe air tanks are overfilled, requiring them to be drained.
- BThe air compressor is operating within acceptable limits.
- CThe air compressor is building pressure too quickly, indicating a governor malfunction.
- DThe air compressor is building pressure too slowly and needs immediate service.
Show answer & explanationAnswer & explanation
Correct answer: B. The air compressor is operating within acceptable limits.
The compressor built pressure from 85 psi to 125 psi in 35 seconds, which is less than the maximum allowed 45 seconds. This indicates the compressor is functioning correctly within specifications.
Why the other options are wrong
- A. The test measures build-up time, not tank capacity; 125 psi is a normal cut-out pressure.
- C. Building pressure quickly is good; a governor malfunction would typically be indicated by incorrect cut-in/cut-out pressures or not stopping compression.
- D. 35 seconds is faster than the 45-second limit, so it's not too slow.
Air Compressor Build-Up Test
This test checks the air compressor's ability to quickly build air pressure from a specified minimum to maximum within a set time limit, ensuring adequate air supply for braking.
- Engine at operating RPM.
- Pressure build-up from 85 to 125 psi.
- Maximum allowable time is typically 45 seconds for combination vehicles.
- Failure indicates a problem with the compressor or leaks.
Memory trick: Compressor's quick build-up, safety's strong hold, less than 45 seconds, a story untold.