Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairMedium
A vehicle with a vacuum power brake booster exhibits a loud hissing sound from the brake pedal area when the pedal is depressed. The pedal feels hard, and engine idle quality noticeably drops when the brakes are applied. What is the MOST likely cause?
- AA leaking master cylinder primary seal.
- BA clogged vacuum hose to the booster.
- CA ruptured power brake booster diaphragm.
- DA faulty booster check valve.
Show answer & explanationAnswer & explanation
Correct answer: C. A ruptured power brake booster diaphragm.
A loud hissing sound from the pedal area when depressed, combined with a hard pedal and a drop in engine idle, strongly indicates a ruptured power brake booster diaphragm. The hiss is atmospheric air being drawn into the engine through the ruptured diaphragm, creating a large vacuum leak that affects engine idle and prevents proper booster operation.
Why the other options are wrong
- A. A leaking master cylinder seal causes a spongy or sinking pedal, not a hiss or engine idle drop.
- B. A clogged vacuum hose would cause a hard pedal due to lack of vacuum, but without a distinct hissing sound from the pedal area or a direct vacuum leak into the engine.
- D. A faulty check valve would cause a hard pedal (no assist) and might prevent the pedal drop test, but typically not a loud hiss from the pedal area or a continuous large vacuum leak affecting idle like a ruptured diaphragm.
Ruptured Booster Diaphragm
A tear or hole in the flexible membrane inside a vacuum brake booster, leading to a vacuum leak, loss of power assist, and often a hissing sound.
- Causes loud hiss from pedal area.
- Results in hard brake pedal.
- Creates a large vacuum leak, affecting engine idle.
Memory trick: Hissing, hard pedal, idle drop? Diaphragm's ruptured, make it stop!