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Difficulty: MediumBraking Systems

While descending a steep mountain road, a driver notices a burning odor and observes that the vehicle takes much longer to stop despite pushing firmly on a high, hard brake pedal. What is the most likely cause of this reduced stopping ability?

  1. A
    Air bubbles compressed within the hydraulic brake lines
  2. B
    A ruptured vacuum booster diaphragm
  3. Overheating of the brake friction linings, leading to brake fadeAnswer
  4. D
    A seized rear drum brake wheel cylinder

Answer

Overheating of the brake friction linings, leading to brake fade
Prolonged downhill braking generates extreme heat between friction linings (brake pads or shoes) and rotors or drums. When temperatures surpass design thresholds, the friction coefficient drops drastically and resins in the friction material can outgas or glaze over. This phenomenon—known as brake fade—causes prolonged stopping distances despite firm pedal pressure.

Step-by-Step Solution

1
Analyze the vehicle operating conditions and observed diagnostic symptoms
The driver is engaged in extended downhill braking, notices a burning smell, experiences extended stopping distances, and encounters a high, hard brake pedal.
A high, firm pedal confirms that hydraulic integrity is intact and that no air is trapped in the fluid lines.
2
Evaluate the mechanical and thermal effects on friction surfaces
Severe continuous braking converts excessive kinetic energy into heat, pushing brake pads and shoes beyond their thermal threshold and causing their friction coefficient to drop sharply.
This reduction in friction effectiveness under extreme heat is defined as brake fade.

Key Concept

Brake Fade and Thermal Limits of Friction Materials
Estimated Time:45s
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Topics

Braking Systems

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