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Question

Difficulty: HardBraking Systems

A vehicle's brake pedal slowly sinks all the way to the floorboard while held at a stoplight, but an inspection reveals no external fluid leaks anywhere in the hydraulic system. What component is most likely failing internally?

  1. A
    Caliper slide pins
  2. Master cylinder piston sealsAnswer
  3. C
    Wheel cylinder piston cups
  4. D
    Drum brake retracting springs

Answer

Master cylinder piston seals
When a vehicle's brake pedal gradually sinks under constant foot pressure without any external loss of brake fluid, the master cylinder's internal rubber piston cups have degraded. This allows pressurized brake fluid to bleed backwards past the piston head into the unpressurized reservoir, collapsing line pressure without leaking outside the unit.

Step-by-Step Solution

1
Analyze the observed symptom
The brake pedal gradually drops to the floorboard under continuous holding force.
A sinking pedal indicates a progressive loss of hydraulic pressure within the closed braking circuit.
2
Evaluate the condition of no external fluid leakage
The fluid volume is retained inside the system rather than escaping via lines, calipers, or wheel cylinders.
External leaks produce visible drops, wet backplates, or a dropping fluid reservoir level; absence of these confirms the failure is entirely internal.
3
Identify the internal component capable of bypassing hydraulic pressure
The master cylinder piston rubber cup seals are allowing fluid to bleed past the piston head back into the unpressurized fluid reservoir chamber.
When internal piston seals deteriorate, fluid slips backwards under steady pedal load, dissipating line pressure while losing no total fluid volume.

Key Concept

Internal Hydraulic Pressure Bypass in Master Cylinders
Estimated Time:1m 0s
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Topics

Braking Systems

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