Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A technician is evaluating a vehicle with a complaint of a 'hard pedal' that requires excessive effort to stop. They connect a vacuum gauge to the brake booster vacuum line and read 18 in. Hg with the engine running at idle. After shutting off the engine, the vacuum gauge reading drops to 0 in. Hg within 30 seconds. What does this test indicate?
- AThe brake booster check valve is faulty.
- BThe brake booster diaphragm is ruptured.
- CThe master cylinder is bypassing internally.
- DThe vacuum supply from the engine is insufficient.
Show answer & explanationAnswer & explanation
Correct answer: A. The brake booster check valve is faulty.
A rapid drop in vacuum after the engine is shut off indicates that the booster is not holding vacuum. Since the engine is supplying adequate vacuum (18 in. Hg is good), the most likely component allowing vacuum to escape is a faulty check valve, which should retain vacuum in the booster.
Why the other options are wrong
- B. A ruptured booster diaphragm would typically cause a hard pedal and a hiss, and the vacuum might not build up adequately in the first place, or it would leak down even with the engine running.
- C. A bypassing master cylinder would cause a soft or sinking pedal, not a hard pedal linked to vacuum loss.
- D. The reading of 18 in. Hg indicates a sufficient vacuum supply from the engine. Insufficient vacuum would be a much lower reading.
Brake Booster Check Valve Test
A brake booster check valve test involves measuring vacuum retention after engine shutdown. A rapid vacuum drop indicates a faulty check valve.
- Check valve holds vacuum in booster.
- Vacuum allows power assist when engine is off.
- Failure causes hard pedal after engine off or during low engine vacuum.
Memory trick: Vacuum drops fast? The check valve's passed its last.