Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A client complains that their vehicle's brake pedal feels 'hard' and requires excessive effort to stop, particularly after sitting for a while. The vacuum hose to the brake booster is correctly attached and shows no visible damage. Which of the following is the most likely cause?
- AA worn master cylinder
- BA leaking brake booster diaphragm
- CA restricted brake line
- DA faulty proportioning valve
Show answer & explanationAnswer & explanation
Correct answer: B. A leaking brake booster diaphragm
A hard brake pedal requiring excessive effort is a primary symptom of a brake booster failure. A leaking diaphragm prevents the booster from holding vacuum, thus not providing power assist.
Why the other options are wrong
- A. A worn master cylinder usually results in a low or sinking pedal, not a hard pedal.
- C. A restricted brake line would typically cause a pull or one wheel to drag, not a hard pedal on all wheels.
- D. A faulty proportioning valve primarily affects rear brake pressure, not overall pedal effort.
Brake Booster Diaphragm Leak
A tear or hole in the rubber diaphragm within the brake booster, preventing it from holding vacuum and providing power assist to the brake pedal.
- Causes hard pedal
- Increases stopping effort
- Often due to age or material degradation
Memory trick: Hard pedal, no vacuum assist, booster's diaphragm is probably missed!