Examkin

Question

Difficulty: Very hardBrake Failure

If a commercial motor vehicle experiences a complete loss of stopping power specifically due to severe thermal brake fade on a long descent, immediately applying the parking brake control valve will effectively stop the vehicle because the mechanical spring brakes operate independently of the pneumatic service brake system.

Answer: Answer

Answer

False
The statement is definitively false because spring brakes utilize the exact same foundation components (drums, shoes, S-cams) as the service brakes. When extreme heat causes the drums to expand beyond the maximum effective stroke of the pushrod, deploying the mechanical spring force cannot overcome the physical gap or the loss of friction material effectiveness.

Step-by-Step Solution

1
Identify the specific root cause of the brake failure described in the scenario.
The failure is explicitly caused by severe thermal brake fade, not an air system failure.
Differentiating between air loss and thermal fade is critical to determining the effectiveness of emergency braking systems.
2
Recall the physical mechanism of thermal brake fade in drum brakes.
Excessive heat causes the brake drums to expand away from the brake shoes, requiring more pushrod travel to make contact, and degrades the friction coefficient of the linings.
Understanding the mechanical reality at the wheel end explains why braking power is lost.
3
Analyze how the parking/spring brake system applies force to the wheels.
When the parking brake valve is pulled, air is exhausted, allowing a heavy mechanical spring to push the pushrod, rotating the slack adjuster and S-cam to press the shoes against the drum.
This confirms that spring brakes share the identical foundation components (shoes, drums, cams) with the service brakes.
4
Synthesize the mechanism of thermal fade with the operation of spring brakes.
Because both systems rely on the same overheated, expanded drums and degraded brake shoes, deploying the spring brakes will not overcome severe thermal fade.
Mechanical independence at the brake chamber does not equate to independent braking surfaces at the wheel.

Key Concept

Foundation Brake Component Sharing and Thermal Fade Limitations
Estimated Time:1m 30s
Rate this question

Topics

Brake Failure

More questions from this topic