CDL General Knowledge TestAir Brakes (if applicable)Hard
A driver is conducting a static air brake leakage test. After building full air pressure, shutting off the engine, and releasing the parking brake, the pressure drops by 5 psi in one minute. The vehicle is a single unit truck. What does this indicate?
- AThere is an excessive air leak in the system.
- BThe air brake system is functioning normally.
- CThe low air pressure warning system is malfunctioning.
- DThe air compressor is faulty and needs replacement.
Show answer & explanationAnswer & explanation
Correct answer: A. There is an excessive air leak in the system.
For a single unit truck, the maximum allowable air loss during a static leakage test (engine off, parking brake released) is 3 psi in one minute. A drop of 5 psi indicates an excessive air leak that must be found and repaired before operating the vehicle.
Why the other options are wrong
- B. A drop of 5 psi in one minute for a single unit is above the maximum allowable 3 psi, indicating a problem.
- C. The warning system's function is to alert at lower pressure (e.g., 60 psi), not to regulate the rate of pressure drop during a static test.
- D. The compressor's function isn't directly tested during a static leak test (engine is off).
Static Air Leakage Test (Single Unit)
A test performed with the engine off and parking brake released to check the integrity of the air brake system for leaks.
- Maximum allowable drop: 3 psi in one minute.
- Must be performed after full air pressure is built.
- Indicates system leaks if pressure drops too quickly.
Memory trick: Three is fine, five is a whine.