Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A vehicle owner reports that their brake pedal feels 'hard' and requires excessive effort to stop the vehicle, particularly when first starting out. The master cylinder reservoir is full, and there are no visible leaks. What component is MOST likely at fault?
- AA faulty brake booster check valve.
- BAir in the brake lines.
- CA seized caliper piston.
- DA clogged compensating port in the master cylinder.
Show answer & explanationAnswer & explanation
Correct answer: A. A faulty brake booster check valve.
A hard brake pedal requiring excessive effort is a classic symptom of a malfunctioning power brake booster. The check valve maintains vacuum in the booster when the engine is off, allowing for 1-2 assisted stops. If the check valve is faulty, the booster loses vacuum, resulting in a hard pedal from the first stop, as there's no power assist.
Why the other options are wrong
- B. Air in the brake lines causes a spongy or soft pedal, not a hard pedal.
- C. A seized caliper piston would typically cause the vehicle to pull to one side or cause excessive drag, not a generally hard pedal for all braking.
- D. A clogged compensating port in the master cylinder would cause residual pressure in the system, leading to dragging brakes or a pedal that does not return fully, not a hard pedal requiring excessive effort.
Brake Booster Check Valve
A one-way valve located in the vacuum line to the brake booster, designed to maintain vacuum within the booster even when the engine is off.
- Ensures power assist for initial brake applications after engine is shut off.
- Failure leads to loss of vacuum in the booster.
- Symptom of failure is a hard brake pedal requiring excessive effort.
Memory trick: Hard pedal? Booster's not boosting, or something's blocking.