Brake Fade on a 6% Grade
You are driving a 2019 Volvo VNL tractor pulling a 53-foot dry van, gross weight 78,000 lb, down a 6% grade that runs for 4 miles. The outside temperature is 95°F. You have been holding 55 mph by keeping steady pressure on the service brake pedal. At mile 2, the pedal feels soft and travels almost to the floor; the truck accelerates to 65 mph. The engine brake is on but set to its lowest setting. A signed escape ramp is 1/2 mile ahead on the right. Traffic is light. Your air pressure gauges still read 110 psi. You smell hot brakes. Your co-driver yells to pump the brakes. The right shoulder is narrow and drops off sharply. The truck has a manual transmission in 9th gear.
1. What is happening to the brakes, and why is pumping the pedal the wrong response?
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This is brake fade from overheating. Continuous light-to-moderate pedal pressure on a long grade builds heat faster than the drums and linings can shed it. The drums expand, the linings lose friction, and the pedal can feel soft or go near the floor even though the air system still has pressure. The gauges reading 110 psi are consistent with fade: this is not an air leak.
Pumping the pedal is the wrong response because it adds more heat and uses more air without restoring friction. It also prevents the brakes from cooling. The co-driver is reacting to the symptom, not the cause. The cause is too much continuous brake work on a grade that should be handled with a lower gear and engine braking.
2. What should you do in the next 30 seconds?
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Act immediately; do not wait to see whether the pedal recovers.
- Downshift to a lower gear to use engine braking and reduce the load on the service brakes.
- Increase the engine brake to its highest setting if the transmission and engine allow it.
- Aim for the escape ramp if you cannot get the speed under control. A 1/2-mile lead is short at 65 mph on a 6% grade, so commit early rather than late.
- Use four-way flashers and the horn to warn traffic.
- If you must use the remaining service brake, use it firmly and briefly rather than riding it, but do not count on it to hold the load.
The escape ramp is the safest choice if speed is still increasing. Trying to "save" the stop by staying on the road risks losing the brakes completely.
3. What should you have done before the grade, and what should you do after stopping?
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Before the grade, you should have selected a low enough gear to hold a safe speed without continuous service braking, turned the engine brake to a high setting, and used snub braking: apply the brakes firmly to reduce speed, then release them and let them cool. You should not have used steady light pedal pressure for miles.
After stopping, keep the brakes applied only as needed, let them cool, and inspect for heat damage, cracked drums, grease or oil from overheated seals, and air leaks. Do not continue down the grade or back onto the road until the brakes have cooled and a proper inspection shows they are working. If the escape ramp was used, follow its instructions and call for recovery.