Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A customer complains of a brake pedal that slowly sinks to the floor when constant pressure is applied, but there are no external leaks visible. A technician suspects an internal master cylinder issue. Which of the following is the MOST likely cause?
- AAir trapped in the brake lines.
- BA faulty brake booster check valve.
- CA clogged brake fluid reservoir filter.
- DWorn secondary master cylinder piston cups.
Show answer & explanationAnswer & explanation
Correct answer: D. Worn secondary master cylinder piston cups.
A slowly sinking brake pedal with no external leaks is a classic symptom of internal master cylinder failure. Specifically, worn or deteriorated secondary piston cups allow fluid to bypass the piston, preventing pressure from being maintained in the hydraulic system.
Why the other options are wrong
- A. Air in the lines typically causes a spongy pedal, which might sink slightly but not consistently or as slowly as an internal leak.
- B. A faulty brake booster check valve affects power assist, leading to a hard pedal, not a sinking pedal.
- C. A clogged reservoir filter would restrict fluid flow, potentially causing a hard pedal or no pedal, not a sinking pedal.
Internal Master Cylinder Leak
A condition where brake fluid bypasses the piston seals within the master cylinder, leading to a loss of hydraulic pressure without external leaks.
- Caused by worn piston cups or bore corrosion.
- Symptom is a slowly sinking brake pedal under constant pressure.
- Requires master cylinder replacement or rebuild.
Memory trick: Sinking pedal? Think 'Internal Leak' or 'External Escape'.