Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairMedium
A technician is diagnosing a spongy brake pedal on a vehicle equipped with a vacuum-assisted power brake system. Initial checks confirm adequate fluid level and no external leaks. The technician then depresses the brake pedal with the engine off several times until the pedal becomes firm. While holding the pedal down, the engine is started. If the pedal does not fall slightly and then hold, which of the following is the MOST likely cause?
- AA faulty power brake booster check valve.
- BA seized master cylinder primary piston.
- CAir in the hydraulic brake lines.
- DA collapsed flexible brake hose.
Show answer & explanationAnswer & explanation
Correct answer: A. A faulty power brake booster check valve.
The described test procedure is used to evaluate the power brake booster's vacuum integrity and operation. If the pedal does not drop slightly when the engine starts, it indicates the booster is not assisting, often due to a failure in maintaining vacuum, such as a faulty check valve.
Why the other options are wrong
- B. A seized master cylinder piston would likely result in a hard pedal or no pedal movement, not specifically prevent the pedal drop with the engine starting.
- C. Air in the lines typically causes a spongy pedal, but the specific test described points to a booster issue, not air.
- D. A collapsed brake hose would restrict fluid flow, causing a hard pedal or pulling, not the absence of a pedal drop during this specific booster test.
Vacuum Booster Check Valve
A one-way valve located on the vacuum booster that maintains vacuum within the booster when the engine is off or manifold vacuum is low, ensuring power assist is available.
- Prevents vacuum loss from the booster.
- Essential for initial brake assist when starting the engine.
- Failure leads to hard pedal or no power assist.
Memory trick: Press, Start, Drop. If no drop, check the valve, then booster's a flop!