CDL General Knowledge TestDriving SafelyHard
A commercial driver is operating a vehicle with air brakes. While performing an air loss test, they notice that with the engine off and the service brakes applied, the air pressure drops more than 4 psi in one minute (for a single vehicle). What does this indicate?
- ANormal air brake system operation.
- BThe air compressor is functioning correctly.
- CThe brake chambers are over-lubricated.
- DThere is an excessive air leak in the system.
Show answer & explanationAnswer & explanation
Correct answer: D. There is an excessive air leak in the system.
For a single commercial vehicle, an air pressure drop of more than 3 psi in one minute with the service brakes released, or more than 4 psi with the service brakes applied, indicates an excessive air leak. This is a critical safety issue as it means the system cannot hold pressure effectively, leading to potential brake failure. Normal operation (A) would show less pressure drop.
Why the other options are wrong
- A. Normal air brake system operation for a single vehicle should show a drop of no more than 3 psi (service brakes released) or 4 psi (service brakes applied) in one minute.
- B. The air compressor's function is related to building pressure, not holding it against a leak when the engine is off.
- C. Brake chamber lubrication issues would affect brake operation, not necessarily the rate of air pressure loss when applied.
Applied Air Loss Test
A test performed on air brake systems to check for excessive air leaks when the service brakes are fully applied.
- Engine off, parking brake set.
- Service brakes fully applied.
- Pressure drop should not exceed 4 psi in one minute for single vehicle.
Memory trick: Press the pedal, watch the gauge, too much drop, danger's stage.