Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A client complains that their brake pedal is firm and high but requires excessive force to stop the vehicle. The technician confirms good brake pad and rotor condition, and the master cylinder is not leaking. What is the MOST likely cause of this symptom?
- AA restricted vacuum supply to the brake booster
- BA faulty proportioning valve
- CAir in the hydraulic system
- DA collapsed flexible brake hose
Show answer & explanationAnswer & explanation
Correct answer: A. A restricted vacuum supply to the brake booster
A firm and high pedal requiring excessive force is a classic symptom of a lack of power assist, which is most commonly caused by a problem with the brake booster or its vacuum supply.
Why the other options are wrong
- B. A faulty proportioning valve would typically cause premature rear wheel lock-up or reduced rear braking, not a firm, high pedal with excessive effort.
- C. Air in the hydraulic system would cause a spongy or soft pedal, not a firm pedal.
- D. A collapsed flexible brake hose would cause a dragging brake or reduced braking on one wheel, not a firm pedal with overall excessive effort.
Brake Booster Failure
A brake booster failure or loss of vacuum supply results in a 'hard pedal' symptom, requiring significantly increased effort to apply the brakes.
- Brake booster multiplies pedal force.
- Relies on engine vacuum (or electric pump).
- Failure causes a firm, high pedal with high effort.
Memory trick: Hard pedal? Think vacuum loss or booster's boss.