Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairEasy
A vehicle with a vacuum-assisted brake booster has a hard brake pedal. The technician observes that the engine idles rough when the brake pedal is depressed. Which of the following is the MOST likely cause?
- AA leaking brake booster diaphragm.
- BA seized brake caliper.
- CA faulty master cylinder check valve.
- DA clogged vacuum filter.
Show answer & explanationAnswer & explanation
Correct answer: A. A leaking brake booster diaphragm.
A leaking brake booster diaphragm introduces an uncontrolled vacuum leak to the engine when the pedal is pressed, causing the engine to run rough due to an unmetered air intake and simultaneously reducing power assist.
Why the other options are wrong
- B. A seized caliper would cause dragging brakes and possibly a pull, but not a rough idle when the pedal is pressed.
- C. A faulty master cylinder check valve typically causes a spongy pedal or a pedal that sinks, not a rough idle.
- D. A clogged vacuum filter would restrict vacuum to the booster, causing a hard pedal, but would not typically cause a rough idle when the pedal is pressed.
Leaking Booster Diaphragm
A rupture or tear in the flexible diaphragm inside a vacuum brake booster, causing a vacuum leak.
- Leads to reduced or no power assist (hard pedal).
- Can cause engine idle issues (rough idle, stalling) if the leak is significant.
- Often accompanied by a hissing sound from the pedal area.
Memory trick: Engine 'sucks' extra air through the leak, making it stumble.