Automobile & Light Truck Certification Test A5 BrakesPower Assist Unit Diagnosis and RepairMedium
During a pre-purchase inspection, a technician notices that the brake pedal on a vacuum-assisted vehicle feels firm with the engine off. When the engine is started, the pedal drops slightly, but then slowly continues to sink towards the floor while steady pressure is maintained. This symptom is MOST indicative of:
- AA leaking vacuum booster check valve.
- BAn internal leak in the master cylinder.
- CA faulty brake pressure differential switch.
- DA restricted brake fluid return line.
Show answer & explanationAnswer & explanation
Correct answer: B. An internal leak in the master cylinder.
The initial slight drop when the engine starts indicates the booster is working correctly and holding vacuum. However, the subsequent slow sinking of the pedal while pressure is maintained is a classic symptom of an internal leak past the primary or secondary seals within the master cylinder, allowing fluid to bypass the pistons.
Why the other options are wrong
- A. A leaking check valve would prevent the initial pedal drop when the engine starts (no assist), or cause a hard pedal, not a slow sink after initial assist.
- C. A faulty pressure differential switch would illuminate a brake warning light, but not cause a sinking pedal.
- D. A restricted return line would cause the brakes to drag or remain applied, not a slowly sinking pedal.
Master Cylinder Internal Leak
A condition where hydraulic fluid bypasses the piston seals within the master cylinder, preventing full pressure buildup and causing the brake pedal to slowly sink.
- Pedal sinks slowly under steady pressure.
- Often no external fluid leaks.
- Indicates worn or damaged piston seals.
Memory trick: Pedal sinks slow, master cylinder's seals let the fluid flow!