Automobile & Light Truck Certification Test A5 BrakesHydraulic System Diagnosis and RepairMedium
A client complains that their brake pedal feels 'soft' and spongy, requiring excessive travel before effective braking occurs. The technician inspects the system and finds no external leaks. The master cylinder reservoir is full. When the technician tries to manually bleed the system, very little fluid comes out of the bleeder screws, even with the pedal fully depressed. Which of the following is the MOST likely cause?
- AA restricted brake line.
- BAir in the ABS hydraulic control unit (HCU).
- CA collapsed brake booster vacuum hose.
- DAn internally faulty master cylinder.
Show answer & explanationAnswer & explanation
Correct answer: D. An internally faulty master cylinder.
If the master cylinder is internally faulty, it may bypass fluid internally, leading to a soft pedal and an inability to build sufficient pressure to effectively push fluid out of the bleeder screws.
Why the other options are wrong
- A. A restricted brake line would cause a hard pedal or no fluid flow to that specific wheel, not a soft pedal with little fluid from all bleeders.
- B. Air in the ABS HCU can cause a spongy pedal but typically wouldn't prevent fluid from exiting the bleeder screws if the master cylinder is functioning.
- C. A collapsed brake booster vacuum hose would lead to a hard pedal, requiring more physical effort, not a soft pedal with little fluid flow.
Internal Master Cylinder Bypass
An internal bypass in a master cylinder occurs when the primary or secondary piston seals fail, allowing brake fluid to leak past them instead of building hydraulic pressure. This results in a soft, spongy pedal and reduced braking effectiveness.
- Pedal feels soft/spongy.
- Fluid bypasses piston seals.
- No external leaks.
- Poor fluid expulsion during bleeding.
Memory trick: When the master cylinder's seals are beat, the brake fluid can't make the pressure meet.