Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairMedium
A customer reports that their vehicle's brake pedal is very hard to push, requiring excessive force to slow down. The engine idle speed is normal, and there are no apparent vacuum leaks. A vacuum gauge connected to the booster hose reads 18 in. Hg with the engine running. What is the MOST likely cause of the hard pedal?
- AA faulty brake booster internal diaphragm.
- BA clogged master cylinder compensation port.
- CWorn front brake pads.
- DA restricted vacuum line to the booster.
Show answer & explanationAnswer & explanation
Correct answer: A. A faulty brake booster internal diaphragm.
If the engine is producing good vacuum (18 in. Hg is healthy), and the vacuum line is not restricted, a hard pedal indicates that the booster itself is not amplifying the force. This often points to an internal diaphragm leak or rupture within the booster, preventing it from holding or utilizing vacuum effectively.
Why the other options are wrong
- B. A clogged compensation port would cause a dragging brake or a pedal that slowly sinks, not a consistently hard pedal with good vacuum.
- C. Worn brake pads would increase pedal travel and potentially require more force, but a 'very hard' pedal usually points to a lack of power assist, which pads don't directly cause.
- D. A restricted vacuum line would lead to low vacuum at the booster, but the gauge reading indicates good vacuum is reaching the booster.
Brake Booster Diaphragm
A flexible rubber membrane inside the vacuum brake booster that separates the vacuum and atmospheric pressure chambers, allowing for pressure differential to assist braking.
- Separates booster chambers.
- Crucial for creating pressure differential.
- Rupture or leak causes loss of assist (hard pedal).
Memory trick: Hard pedal, good vacuum? The booster's diaphragm is probably undone!