Brake Fade on the I-40 Descent East of Albuquerque
You are driving a 2018 Kenworth T680 with a 53-foot refrigerated trailer, gross weight 79,500 lb, westbound on I-40 approaching the 6-mile, 6% descent east of Albuquerque. It is 96°F. You are in 9th gear of a 10-speed manual at 55 mph with the engine brake on high. The engine brake is holding 55 mph, but traffic slows abruptly; you apply the service brakes firmly for about 10 seconds, then release. Within two minutes you smell hot brake lining and the pedal now travels farther before the truck slows. The air gauge remains at 110 psi, and the low-air warning does not sound. The next exit is 2 miles ahead, and there is a runaway truck ramp 1.5 miles ahead.
1. What is happening to the brakes, and why did the pedal change if system air pressure is steady?
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The steady 110 psi gauge rules out an air leak. What changed is heat. Repeated or prolonged service brake application on a 6% grade with 79,500 lb has overheated the drums and linings. As lining temperature rises, friction falls, so the pedal must travel farther and the truck slows less for the same effort. That is brake fade. Drum expansion also increases pushrod travel, which adds to the long-pedal feel. The engine brake was helping, but the 10-second service brake application added a large amount of heat quickly. I would not mistake this for a compressor or leak problem. The fix is to reduce heat input, not to pump the pedal or keep applying more pressure.
2. What should you do immediately, and what braking technique should have been used on this descent?
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Immediately: downshift to a lower gear, keep the engine brake on high, and brake in short, firm applications rather than continuous pressure. If speed cannot be controlled below about 40 mph or the pedal continues to fade, use the runaway ramp 1.5 miles ahead; do not pass it hoping for the exit. If the truck is still controllable, take the exit in 2 miles and stop away from traffic. The correct technique before the grade is to select a low enough gear, let the engine brake do most of the work, and use snub braking: apply the service brakes firmly for a few seconds to bring speed down, then release and let them cool. Riding the brakes keeps them hot and guarantees fade. Once stopped, set the spring brakes, chock the wheels, and let the brakes cool before continuing.