CDL Air Brakes TestUsing Air BrakesMedium
During a pre-trip inspection, a driver tests the service brakes on a commercial vehicle with air brakes by applying full pressure for one minute. The air pressure drop should not exceed a certain amount. For a combination vehicle, what is the maximum allowable air pressure drop in one minute?
- A4 psi.
- B5 psi.
- C3 psi.
- D2 psi.
Show answer & explanationAnswer & explanation
Correct answer: A. 4 psi.
For a combination vehicle, the maximum allowable air pressure drop during a one-minute service brake test (with the engine off and brakes fully applied) is 4 psi. A drop greater than this indicates a leak that needs to be addressed.
Why the other options are wrong
- B. 5 psi is too high and would indicate a significant leak that requires immediate repair.
- C. 3 psi is not the correct maximum for any standard air brake leakage test.
- D. 2 psi is the maximum allowable drop for a single unit (straight truck or bus).
Air Brake Leakage Test (Combination)
A pre-trip inspection procedure for air-braked combination vehicles to check for air system leaks by measuring pressure drop over one minute with brakes fully applied.
- Engine off, service brakes fully applied.
- Maximum 4 psi pressure drop in one minute.
- Higher drop indicates a leak.
- Crucial for safety and compliance.
Memory trick: Combo Four, Single Two, Leaks are bad, it's true!