Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairHard
A technician is performing a brake system inspection on a vehicle with a 'soft' brake pedal. The master cylinder reservoir is full, and there are no external leaks. When the technician depresses the pedal, it slowly sinks to the floor even with steady pressure. Which of the following is the MOST likely cause?
- AA collapsed flexible brake hose.
- BAir trapped in the hydraulic system.
- CA faulty brake booster check valve.
- DAn internal leak in the master cylinder.
Show answer & explanationAnswer & explanation
Correct answer: D. An internal leak in the master cylinder.
A pedal that slowly sinks to the floor under steady pressure, with a full reservoir and no external leaks, is the classic symptom of an internal master cylinder leak. This occurs when fluid bypasses the primary cup seal within the master cylinder.
Why the other options are wrong
- A. A collapsed flexible brake hose would cause a drag or pull to one side, not a sinking pedal.
- B. Air in the system causes a spongy pedal that may firm up with pumping, not one that slowly sinks.
- C. A faulty brake booster check valve would cause a hard pedal, not a sinking pedal.
Internal Master Cylinder Leak
A condition where brake fluid bypasses the piston seals within the master cylinder, preventing pressure buildup and causing the brake pedal to slowly sink.
- Pedal sinks slowly under steady pressure.
- No external fluid loss.
- Caused by worn primary cup seal.
Memory trick: Pedal sinks slow, master cylinder's flow, seals won't go.