Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairMedium
A technician is performing a road test on a vehicle with a vacuum-assisted brake booster. The customer complains that the brakes feel normal initially, but after several rapid applications, the pedal becomes hard. Which of the following is the MOST likely cause?
- AAn internally leaking power brake booster.
- BA faulty ABS pump motor.
- CA leaking master cylinder primary seal.
- DA partially restricted vacuum hose to the booster.
Show answer & explanationAnswer & explanation
Correct answer: D. A partially restricted vacuum hose to the booster.
If the booster provides assist initially but loses it after rapid applications, it suggests that the booster isn't able to replenish its vacuum quickly enough. A partially restricted vacuum hose would limit the flow of vacuum, causing the booster to 'run out' of assist after repeated use, as it can't draw sufficient vacuum fast enough.
Why the other options are wrong
- A. An internally leaking booster would likely cause a hard pedal consistently, or a noticeable hiss, not just after rapid applications.
- B. A faulty ABS pump motor would affect ABS operation, not the basic power assist function leading to a hard pedal after rapid applications.
- C. A leaking master cylinder primary seal would typically cause a spongy or slowly sinking pedal, not a hard pedal after rapid applications.
Booster Vacuum Restriction
Any blockage or kink in the vacuum line supplying the brake booster, which limits the rate at which vacuum can be drawn into the booster.
- Restricts vacuum flow to booster.
- Causes loss of assist after rapid pedal applications.
- Can be due to collapsed hose or carbon buildup.
Memory trick: Rapid pumps, hard pedal? Vacuum hose restriction, don't meddle!