Brake Fade on a 6% Grade
You drive a 33,000 lb GVWR straight truck with air drum brakes, loaded to 32,000 lb, descending a 6% grade for 3 miles. You are in 5th gear at 50 mph, using the service brake in short applications, but traffic slows and you hold the brake longer. After 1.5 miles, you smell hot brake lining, the pedal feels low and spongy, and the truck does not slow as expected when you apply the brake. Both air gauges remain at 105 psi. The yellow spring brake indicator is off. There is a runaway truck ramp 1/2 mile ahead on the right. You must decide what is causing the problem, what to do immediately, and whether the parking brake is an option.
1. What is happening, and what should you do in the next few seconds?
Read the worked answer
This is brake fade: the drum brakes are overheating from continuous or repeated use on the grade, so friction drops and the pedal may feel low or spongy. Air pressure is not the problem because both gauges are at 105 psi. Immediately downshift to a lower gear to use engine braking, stop using the brake in long continuous applications, and use short, firm applications only as needed. If the truck will not slow and the ramp is available, take the runaway ramp; do not wait until the brakes are smoking. Keep the steering controlled and use flashers. Do not pump the pedal repeatedly in a panic; that adds heat and uses air without solving the fade.
2. Why should you not use the parking brake or spring brake to stop the truck at 50 mph?
Read the worked answer
The parking brake on an air-brake truck is typically the spring brake. Applying it at highway speed can lock the rear wheels, cause a skid or jackknife, and it may not provide progressive control. It is an emergency and parking device, not a service brake. It also may not stop the truck any faster once the service brakes are faded, and it can damage the brake system. The better options are downshifting, using the escape ramp, and finding a safe place to stop. If the service brakes fail completely, you may have to use the emergency feature, but that is a last resort at low speed, not a normal downhill control method.
3. After you stop, what must be checked before continuing?
Read the worked answer
Stop in a safe place, set the parking brake, and let the brakes cool; do not pour water on hot drums. Check for smoke, fluid leaks, and damaged lines. After cooling, inspect brake adjustment and lining condition, and make a low-speed test application in a safe area. If the pedal is still low, the truck pulls, or the brakes smell hot again, do not continue; call for repair. Before the next descent, select a lower gear at the top of the grade, keep speed low, and use the brakes in short applications rather than riding them. The fact that air pressure stayed normal does not clear the vehicle, because the failure was thermal, not an air leak.