CDL Air Brakes TestAir Brake System PartsEasy
A driver is performing a pre-trip inspection and notices that the air pressure in the system consistently builds from 85 psi to 125 psi in 55 seconds. What does this indicate about the air compressor's performance?
- AThe air compressor is building pressure too quickly, indicating a problem.
- BThe air compressor governor is set incorrectly.
- CThe air compressor is building pressure at an acceptable rate.
- DThe air compressor is building pressure too slowly, requiring inspection.
Show answer & explanationAnswer & explanation
Correct answer: C. The air compressor is building pressure at an acceptable rate.
According to CDL regulations, the air compressor should build air pressure from 85 psi to 125 psi within 45 seconds in dual air systems. A build-up time of 55 seconds is within acceptable limits for many commercial vehicles, especially with larger air systems or older compressors, where up to 60 seconds is often considered acceptable.
Why the other options are wrong
- A. Building pressure too quickly is rarely a compressor issue; it's usually a gauge or system leak issue if it's not maintaining pressure.
- B. An incorrect governor setting would affect the cut-in/cut-out pressures, not the rate of pressure build-up between those points.
- D. Building pressure too slowly would be indicated by a time exceeding the regulatory limit, which is not the case here.
Air Compressor Build-Up Rate (CDL)
The time it takes for the air compressor to increase air pressure from 85 psi to 125 psi, which must be within specific regulatory limits (typically 45 seconds or less for dual systems).
- Tested during pre-trip inspection.
- Indicates compressor health and system leaks.
- Failure to meet minimum rate requires repair.
- Specific times can vary slightly by vehicle and jurisdiction (e.g., 45 sec for dual, 60 sec for older/larger).
Memory trick: Pressure climbing, time is ticking, safety's on the line.