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Difficulty: HardBrake Failure

A commercial combination vehicle equipped with S-cam drum brakes is navigating repeated stop-and-go decelerations in heavy traffic. The driver applies the service brake pedal and discovers that the vehicle fails to decelerate effectively, despite both primary and secondary air gauges reading a normal 120 psi120\text{ psi}. Which mechanical condition is responsible for this loss of braking force, and what is the proper emergency response?

  1. Thermal expansion has caused the brake drums to expand away from the shoes beyond available pushrod travel; the driver should downshift to use engine braking and look for an escape route or safe stopping area.Answer
  2. B
    Pushrod travel limits were exceeded due to drum expansion; the driver should rapidly fan the foot valve to pump additional reservoir air into the chambers and force lining contact.
  3. C
    Brake linings have overheated and glazed; the driver should immediately activate maximum engine retarder braking while steering abruptly onto the shoulder to induce wheel drag.
  4. D
    Air line transmission lag has increased total stopping distance; the driver should shift the transmission into neutral and hold the service brake pedal continuously to the floor.

Answer

Thermal expansion causes the brake drums to expand away from the shoes beyond available pushrod travel; the driver should downshift to utilize engine braking and steer toward an escape route or safe stopping area.
Thermal brake fade occurs when excessive friction generates high temperatures that cause the brake drums to expand outward away from the brake shoes. As the drums expand, the pushrods must travel farther to establish contact, eventually bottoming out at their maximum stroke limit. When this happens, braking force drops drastically even though air pressure gauges show normal operating levels. The driver must immediately downshift to engage engine compression braking and guide the vehicle toward an escape route or safe deceleration area.

Step-by-Step Solution

1
Analyze the mechanical failure mechanism given normal air pressure
Identify that full air pressure (120 psi120\text{ psi}) rules out pneumatic supply failure, indicating thermal brake fade where heat expansion of the drum exceeds pushrod stroke limits.
When brake drums become excessively hot from repeated heavy braking, they expand outward. If the pushrod reaches its maximum stroke limit in the chamber, the shoes cannot exert sufficient force against the drum surface.
2
Determine the correct immediate tactical response
Downshift the transmission into a lower gear to maximize engine compression braking and locate an escape ramp or safe path off the roadway.
Engine braking provides deceleration without relying on the overheated foundation brakes, and identifying an escape path prevents high-speed collisions.
3
Evaluate and eliminate hazardous emergency actions
Reject fanning the brakes (depletes air), shifting to neutral (removes engine retardation), or abrupt maneuvers with retarders (causes wheel skids).
Emergency actions must preserve vehicle stability and utilize alternative retardation methods safely.

Key Concept

Brake Fade and Pushrod Stroke Limits
Estimated Time:1m 30s
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Topics

Brake Failure

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