CDL General Knowledge TestCombination Vehicles (if applicable)Medium
A driver is performing a pre-trip inspection on a combination vehicle's air brake system. After building full air pressure, the driver applies the service brake and holds the pedal down. For a tractor-trailer combination, what is the maximum acceptable air pressure drop in one minute for the service brakes?
- A4 psi
- B3 psi
- C1 psi
- D2 psi
Show answer & explanationAnswer & explanation
Correct answer: A. 4 psi
For a tractor-trailer combination, with the engine off, full air pressure built, and the service brake pedal fully depressed, the air pressure drop should not exceed 4 psi in one minute. A greater drop indicates excessive air leaks that need to be addressed before operation.
Why the other options are wrong
- B. 3 psi is not the correct standard for a combination vehicle service brake test.
- C. 1 psi is typically the maximum for a single vehicle (non-combination).
- D. 2 psi is not the correct standard for a combination vehicle service brake test.
Combination Vehicle Air Brake Leakage Rate
The maximum allowable air pressure drop when the service brakes are applied on a tractor-trailer combination is 4 psi in one minute, indicating the integrity of the air brake system.
- Measured with engine off, full air pressure, service brake applied.
- Higher drop indicates leaks.
- Critical safety check for air brake systems.
Memory trick: Four for the floor, one for the single, three for the parking, and then you're ready to mingle.