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Question

Difficulty: MediumBrake Failure

After observing burning lining odor and diminishing deceleration on a continuous mountain downgrade, a commercial driver determines that the vehicle is suffering from severe thermal brake fade. If a runaway vehicle ramp is not immediately accessible, which immediate action should the driver take to control speed and bring the vehicle to a halt?

  1. A
    Pump the brake pedal rapidly to rebuild hydraulic pressure and cool the friction surfaces.
  2. Shift the transmission down into a lower gear to maximize engine braking and look for an escape route.Answer
  3. C
    Shift the transmission into neutral and rely solely on rolling friction and increased following distance to stop.
  4. D
    Maintain continuous heavy pedal pressure using repeated snub cycles to force the drums into full contact.

Answer

Shift the transmission down into a lower gear to maximize engine braking and look for an escape route.
When brake fade occurs on a downgrade, the friction surfaces have overheated and expanded, making normal braking ineffective. The driver's immediate priority is to downshift into a lower gear while vehicle speed still permits gear engagement. This utilizes the engine's compression to resist downhill momentum while the driver actively seeks an escape path or truck ramp.

Step-by-Step Solution

1
Identify the nature of the failure as thermal brake fade caused by overheated brake drums expanding away from the shoes.
Recognize that service brakes cannot provide sufficient mechanical friction to stop the vehicle on the descent.
Attempting to force overheated service brakes to work creates more heat and worsens the expansion.
2
Engage engine braking by shifting into a lower gear as early as possible before speed increases excessively.
Engine compression retards the drive wheels, resisting gravity and helping keep vehicle speed under control.
If vehicle speed becomes too high, matching engine RPM to road speed becomes impossible, locking the transmission out of lower gears.
3
Scan ahead for escape routes, such as runaway vehicle ramps, open flat ground, or an upward-sloping roadside.
The vehicle can be guided off the roadway safely to come to a complete stop without rolling over or colliding with traffic.
An alternative stopping path must be utilized when service brakes are thermally disabled.

Key Concept

Emergency response to thermal brake fade on downgrades
Estimated Time:1m 0s
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Topics

Brake Failure

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