Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairEasy
A technician is diagnosing a vehicle with a spongy brake pedal that gradually firms up after several rapid pumps. There are no visible external leaks. Which of the following is the MOST likely cause?
- AAir trapped in the hydraulic system
- BA seized caliper piston
- CA faulty proportioning valve
- DA restricted brake line
Show answer & explanationAnswer & explanation
Correct answer: A. Air trapped in the hydraulic system
A spongy brake pedal that improves with pumping is a classic symptom of air in the hydraulic system. Air is compressible, leading to a soft pedal, but pumping can temporarily build pressure.
Why the other options are wrong
- B. A seized caliper piston would typically cause drag or a pull, not a spongy pedal that firms up.
- C. A faulty proportioning valve primarily affects rear brake pressure and would not typically cause a spongy pedal that firms up with pumping.
- D. A restricted brake line would typically cause a hard pedal or a pull to one side, not a spongy pedal that improves with pumping.
Air in Brake System
Air trapped within the hydraulic brake fluid, which is compressible unlike fluid, leading to a soft or spongy brake pedal feel.
- Causes a spongy or soft pedal.
- Pedal feel may improve with rapid pumping.
- Requires bleeding the brake system to remove.
Memory trick: Soft pedal, puff of air, pump to firm it fair.