Automobile & Light Truck Certification Test A5 BrakesDisc Brake Diagnosis and RepairHard
After performing a brake job that included new pads and rotors, a technician notices that the brake pedal requires excessive force to stop the vehicle, even though the pedal feel is firm. The vehicle previously had good braking performance. What is the MOST likely cause?
- AOver-torqued lug nuts.
- BA faulty vacuum booster check valve.
- CResidual air in the brake lines.
- DImproperly installed brake pad shims.
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty vacuum booster check valve.
A firm pedal with excessive force required to stop indicates a lack of power assist. Since the brake job was just completed, a likely culprit for loss of power assist is an issue with the vacuum booster, such as a faulty check valve preventing proper vacuum retention, or a vacuum leak.
Why the other options are wrong
- A. Over-torqued lug nuts can lead to rotor warpage or wheel stud damage, but not directly a hard pedal requiring excessive force for braking.
- C. Residual air would cause a soft or spongy pedal, not a firm pedal requiring excessive force.
- D. Improperly installed shims would typically cause noise or uneven wear, not a firm pedal with reduced stopping power.
Power Brake Assist
A system (typically vacuum or hydraulic) that multiplies the force applied by the driver to the brake pedal, making braking easier.
- Vacuum boosters are most common.
- Loss of assist results in a 'hard' pedal and increased stopping distance.
- Relies on a vacuum source (engine or dedicated pump).
Memory trick: Hard pedal, no vacuum medal.