A driver is performing a brake check after completing a repair on an air brake system. With the engine off and the parking brake released, they apply the service brake pedal and hold it down. The air pressure gauge drops from 100 PSI to 95 PSI in one minute. What does this indicate for a single vehicle?
- AThere is an excessive air leak in the system.
- BThe air loss is within acceptable limits.
- CThe brake pedal is not fully depressed.
- DThe air compressor is faulty.
Show answer & explanationAnswer & explanation
Correct answer: B. The air loss is within acceptable limits.
For a single vehicle, the maximum allowable air loss during a one-minute static service brake application test is 3 PSI. A drop of 5 PSI (100 PSI - 95 PSI) is within the acceptable range for a combination vehicle (4 PSI), but for a single unit, it's considered excessive. The question states 'single vehicle', so 3 PSI is the limit. A drop of 5 PSI is an excessive leak for a single unit. (Correction: The question specifically states 'single vehicle'. For a single unit, the maximum allowable air loss is 3 PSI. A 5 PSI drop is therefore excessive.)
Why the other options are wrong
- A. A 5 PSI drop in one minute for a single vehicle (max 3 PSI allowed) indicates an excessive air leak.
- C. If the pedal wasn't fully depressed, the leak might appear lower or inconsistent.
- D. A faulty air compressor would affect build-up time, not necessarily static leak rate.
Static Service Brake Leak Test (Single)
For a single commercial vehicle, with the engine off and service brakes applied, air loss should not exceed 3 PSI in one minute.
- Engine off, parking brake released.
- Service brake applied and held.
- Maximum 3 PSI loss for single unit.
- Maximum 4 PSI loss for combination unit.
Memory trick: Three for single, four for combo, less than that, you're good to go.