Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is performing a vacuum check on a brake booster. They pump the brake pedal several times with the engine off to deplete any residual vacuum, then hold the pedal down while starting the engine. What should happen to the brake pedal if the booster is functioning correctly?
- AThe pedal should slowly rise as vacuum builds.
- BThe pedal should become hard and difficult to depress further.
- CThe pedal should remain firm and not move.
- DThe pedal should drop slightly as vacuum assist becomes available.
Show answer & explanationAnswer & explanation
Correct answer: D. The pedal should drop slightly as vacuum assist becomes available.
When the engine starts, vacuum is supplied to the booster. If the booster is working correctly, it will apply assist, causing the pedal to drop slightly under the technician's foot as the vacuum helps push the master cylinder piston.
Why the other options are wrong
- A. The pedal should drop, not rise, as assist is applied.
- B. This indicates a completely failed booster or no vacuum, as no assist is provided, making the pedal harder.
- C. This indicates a faulty booster or lack of vacuum, as no assist is provided.
Brake Booster Vacuum Test
A diagnostic procedure to check the functionality of a vacuum brake booster by observing pedal movement when the engine is started with the pedal depressed.
- Deplete vacuum with engine off
- Hold pedal, start engine
- Pedal should drop slightly if functional
Memory trick: Engine on, pedal drops, booster's working, the assist never stops!