Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a vacuum-assist brake booster. The customer reports that the brake pedal is hard to push after the engine has been turned off for an hour, but feels normal when the engine is running. Which of the following is the most likely cause?
- AA restricted vacuum hose to the booster.
- BA leaking check valve in the vacuum booster.
- CA faulty master cylinder.
- DA collapsed flexible brake hose.
Show answer & explanationAnswer & explanation
Correct answer: B. A leaking check valve in the vacuum booster.
A leaking check valve in the vacuum booster allows air to enter the booster, causing the vacuum reserve to dissipate when the engine is off. This results in a hard pedal feel until the engine is started and vacuum is restored. The other options would typically cause issues while the engine is running or manifest differently.
Why the other options are wrong
- A. A restricted vacuum hose would typically cause a hard pedal even when the engine is running, as vacuum would not be adequately supplied.
- C. A faulty master cylinder often results in a spongy or slowly sinking pedal, not typically a hard pedal after the engine is off.
- D. A collapsed flexible brake hose would cause a restricted fluid flow, leading to a hard pedal and possibly pulling to one side, but not specifically after the engine is off.
Vacuum Booster Check Valve Function
The vacuum booster check valve holds vacuum in the booster when the engine is off, ensuring power assist for a few brake applications after shutdown.
- Located on the vacuum booster where the hose connects.
- Allows air to be drawn out of the booster but prevents it from re-entering.
- Failure leads to loss of vacuum reserve when engine is off.
Memory trick: Vacuum valve's seal, ensures the pedal's feel, even when the engine's off, it's a critical deal.