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Difficulty: MediumBrake Failure

If an air brake-equipped commercial vehicle suffers total service brake failure on a long downgrade due to excessive heat and thermal brake fade, engaging the parking brake control valve will reliably stop the vehicle because emergency spring brakes apply pressure to a separate, independent set of friction drums.

Answer: Answer

Answer

False
The statement is false because emergency spring brakes apply mechanical force through the same S-cams, shoes, linings, and drums as the service brakes. When severe thermal fade occurs on a downgrade, the drums expand outward and the linings glaze. Consequently, releasing the air to apply the spring brakes cannot create effective friction against the overheated, expanded drums to reliably halt the vehicle.

Step-by-Step Solution

1
Analyze the mechanical mechanism of thermal brake fade during downhill operation
Identify that excessive braking generates extreme heat, causing brake drums to expand radially and friction linings to glaze, which requires extended pushrod stroke and dramatically reduces stopping torque.
Understanding why the service brakes failed initially is essential to determine whether secondary mechanical application will function.
2
Evaluate the foundation components shared between service brakes and spring brakes
Determine that spring brakes do not have independent friction surfaces; they utilize the same mechanical linkage, S-cams, shoes, linings, and drums on the drive axles.
Commercial motor vehicle air brake design integrates the spring brake chamber onto the same foundation wheel assembly as the service brake chamber.
3
Assess the effectiveness of spring brake actuation under severe thermal fade conditions
Conclude that applying the parking/emergency brake valve will not reliably stop the runaway vehicle because the friction surfaces are already expanded and thermally compromised.
Mechanical spring force cannot overcome the physical expansion of overheated drums and glazed friction material, making the statement false.

Key Concept

Shared foundation friction components between service and emergency spring brakes during thermal brake fade
Estimated Time:1m 0s
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Topics

Brake Failure

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