Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A technician is performing a diagnostic on a vehicle with a complaint of a soft brake pedal and excessive travel. During inspection, they find that the brake fluid reservoir is consistently low, requiring frequent topping off. There are no visible external leaks from the lines, calipers, or master cylinder. What is the MOST likely cause of the recurring low fluid level and soft pedal?
- AAn internal leak in the master cylinder
- BWorn rear drum brake shoes requiring adjustment
- CA leaking wheel cylinder that is not externally visible
- DA faulty brake booster check valve
Show answer & explanationAnswer & explanation
Correct answer: C. A leaking wheel cylinder that is not externally visible
A leaking wheel cylinder, particularly in drum brakes, can leak fluid internally onto the brake shoes and drum, where it is absorbed or quickly evaporates, making the leak not externally visible but still causing a fluid loss and soft pedal.
Why the other options are wrong
- A. An internal leak in the master cylinder would cause a sinking pedal but typically would not result in a loss of fluid from the reservoir, as the fluid is just bypassing internally.
- B. Worn rear drum brake shoes requiring adjustment would cause excessive pedal travel, but not a loss of brake fluid from the system.
- D. A faulty brake booster check valve would cause a hard pedal, not a soft pedal or fluid loss.
Invisible Wheel Cylinder Leak
A wheel cylinder can leak internally onto brake shoes within a drum brake, making the fluid loss not externally visible, but still causing a low fluid level and soft pedal.
- Fluid absorbed by shoes or evaporates.
- Causes fluid loss and can introduce air.
- Requires drum removal for inspection.
Memory trick: Hidden fluid loss? Check the drum's inner boss.