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. After checking for external leaks and bleeding the system conventionally with no improvement, the technician suspects a specific issue within the master cylinder. Which of the following is the MOST likely cause?
- AA restricted brake line to one of the calipers.
- BAn internal bypass of fluid past the primary piston seal.
- CA faulty check valve in the brake booster.
- DA seized proportioning valve.
Show answer & explanationAnswer & explanation
Correct answer: B. An internal bypass of fluid past the primary piston seal.
A spongy pedal that firms up after multiple pumps is a classic symptom of an internal master cylinder leak or bypass, where fluid is leaking past the piston seals within the master cylinder, requiring multiple strokes to build pressure.
Why the other options are wrong
- A. A restricted brake line would typically cause a dragging brake or reduced braking on one wheel, not an overall spongy pedal that pumps up.
- C. A faulty check valve in the brake booster would cause a hard pedal, not a spongy pedal that pumps up.
- D. A seized proportioning valve would affect front-to-rear brake balance, potentially causing premature lock-up or reduced braking at one end, not a spongy pedal that pumps up.
Master Cylinder Internal Bypass
An internal bypass in the master cylinder occurs when fluid leaks past the piston seals, preventing immediate pressure buildup and causing a spongy pedal that may firm up after multiple pumps.
- Pedal sinks slowly or feels spongy.
- Often improves with rapid pumping.
- No external fluid loss typically.
Memory trick: Pumping up a spongy pedal points to a master cylinder's internal seal's downfall.