Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a vacuum power brake booster. The customer reports that the brake pedal feels hard and requires excessive effort to stop the vehicle, especially after making several stops in traffic. Testing reveals that engine vacuum at idle is 18 inHg, but after 3-4 rapid pedal applications, the vacuum drops to 5 inHg and recovers slowly. Which of the following is the MOST likely cause?
- AA leaking brake caliper.
- BA restricted vacuum hose to the booster.
- CA faulty master cylinder.
- DA malfunctioning ABS hydraulic unit.
Show answer & explanationAnswer & explanation
Correct answer: B. A restricted vacuum hose to the booster.
A restricted vacuum hose would limit the booster's ability to replenish vacuum quickly after multiple brake applications, leading to a hard pedal. The engine is producing sufficient vacuum, indicating the issue is between the engine and the booster.
Why the other options are wrong
- A. A leaking caliper would cause a loss of brake fluid and a spongy pedal, not a hard pedal due to power assist issues.
- C. A faulty master cylinder typically causes a spongy pedal or a pedal that slowly sinks, not a hard pedal due to insufficient assist.
- D. An ABS hydraulic unit malfunction usually results in ABS warning lights, erratic braking, or a spongy pedal, not a hard pedal from insufficient vacuum assist.
Vacuum Booster Restriction
A condition where the vacuum supply to the brake booster is impeded, leading to reduced power assist and a hard brake pedal.
- Causes increased pedal effort, especially after multiple applications.
- Can be due to a collapsed or kinked vacuum hose, or a faulty check valve.
- Engine vacuum may be good, but booster vacuum recovery is slow.
Memory trick: Hard pedal, low vacuum recovery means a 'tight' vacuum line.