Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a spongy brake pedal that gradually firms up after several rapid pumps. There are no external leaks, and the master cylinder reservoir is full. Which of the following is the MOST likely cause of this symptom?
- AAn internal leak in the master cylinder secondary cup.
- BAir trapped in the hydraulic system.
- CA restricted brake line.
- DA faulty brake booster check valve.
Show answer & explanationAnswer & explanation
Correct answer: B. Air trapped in the hydraulic system.
A spongy pedal that firms up with multiple pumps is a classic symptom of air in the hydraulic system. Air is compressible, so the first pump compresses the air, and subsequent pumps build pressure, giving a firmer pedal. If the air is not completely expelled, the pedal will eventually become spongy again.
Why the other options are wrong
- A. An internal master cylinder leak (often the primary cup) causes the pedal to slowly sink, not firm up with pumping.
- C. A restricted brake line would likely cause a hard pedal or pulling to one side, not a spongy pedal that firms up.
- D. A faulty brake booster check valve would result in a hard pedal due to loss of power assist, not a spongy pedal.
Air in Brake System
Air trapped in the hydraulic brake system is compressible, leading to a 'spongy' pedal feel that often improves temporarily with repeated pumping as the air is compressed.
- Air is highly compressible, unlike brake fluid.
- First pedal pump compresses air, second pump builds pressure.
- Causes a soft, spongy, or low pedal.
- Requires bleeding the system to remove all air.
Memory trick: Spongy and pumpable? Air's your problem.