CDL Air Brakes TestInspecting Air Brake SystemsMedium
A driver is performing a pre-trip inspection on a tractor-trailer combination. After charging the air system to 125 psi, they shut off the engine, release the parking brake, and chock the wheels. They then fully apply the service brake pedal for one minute. The air pressure drops from 125 psi to 118 psi. What action should the driver take?
- APump the brake pedal several times to reseat the brake valves and retest.
- BImmediately inspect for leaks and repair the air brake system before driving.
- CProceed with the trip, as this is within acceptable limits for a combination vehicle.
- DRecord the observation and recheck at the next stop.
Show answer & explanationAnswer & explanation
Correct answer: B. Immediately inspect for leaks and repair the air brake system before driving.
For a combination vehicle, the maximum allowable air pressure drop during an applied air leak test is 4 psi in one minute. A drop of 7 psi (125 - 118 = 7) is excessive and indicates a significant leak that must be found and repaired before the vehicle is operated.
Why the other options are wrong
- A. Pumping the pedal will not fix a leak and could further deplete air pressure, making the situation worse.
- C. The acceptable limit for a combination vehicle is 4 psi, so 7 psi is not acceptable.
- D. This is a safety-critical defect; recording and rechecking later is insufficient and unsafe.
Applied Air Leak Test (Combination Vehicle)
This test checks for air leaks in the service brake system of a combination vehicle by applying the brakes and monitoring pressure loss.
- Engine off, parking brake off, wheels chocked.
- Fully charge air system.
- Apply service brake fully and hold for one minute.
- Pressure drop should not exceed 4 psi for a combination vehicle.
Memory trick: Combined, a 4 psi drop is fine; more means trouble down the line.