Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a severely 'hard' brake pedal and significantly reduced braking effectiveness. The engine is running, and there are no external fluid leaks. The technician performs a vacuum test on the brake booster and finds consistent vacuum output from the engine, but the booster itself does not hold vacuum when the engine is turned off. What is the MOST likely cause of this condition?
- AA collapsed flexible brake hose.
- BA ruptured brake booster diaphragm.
- CA faulty master cylinder primary cup seal.
- DAn obstructed brake line.
Show answer & explanationAnswer & explanation
Correct answer: B. A ruptured brake booster diaphragm.
A hard brake pedal with the engine running indicates a lack of power assist. If the engine provides vacuum but the booster doesn't hold it, it points to an internal leak, most commonly a ruptured diaphragm, preventing the booster from creating a pressure differential.
Why the other options are wrong
- A. A collapsed hose would cause a restriction and a soft pedal, not a hard pedal.
- C. A faulty master cylinder primary cup seal would cause a sinking pedal, not a hard pedal.
- D. An obstructed brake line would cause reduced braking on one wheel or circuit, not a hard pedal for the entire system.
Brake Booster Diaphragm Rupture
A ruptured brake booster diaphragm prevents the booster from holding vacuum and creating the necessary pressure differential to assist braking, resulting in a 'hard' brake pedal.
- Brake boosters use engine vacuum to assist braking.
- The diaphragm separates atmospheric pressure from vacuum.
- A rupture allows vacuum to escape, neutralizing the assist.
- Symptom: hard brake pedal requiring excessive force.
Memory trick: Hard pedal + no vacuum hold = a 'hole' in the booster.