Brake Fade on a Long Interstate Descent
A driver of a 2021 Peterbilt 579 with a 53-foot refrigerated trailer, gross 80,000 lb, descends a 6% grade for 4 miles on I-80 near Bloomsburg. The driver wants to hold 55 mph and uses light, continuous brake pressure. Halfway down, the driver notices a burning smell and the service brake pedal feels normal, but the truck does not slow as it did earlier. The air pressure gauges read 115 psi primary and 113 psi secondary. The engine brake is off, and the transmission is in 10th gear. The driver sees a sign for a rest area 0.5 mile ahead. Outside temperature is 85°F, and no warning lights are on.
1. What is the likely cause of the reduced braking, and why are the air pressure gauges still normal?
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The likely cause is brake fade from overheating. The driver has been using light, continuous brake pressure for a long descent, which builds heat in the drums and linings without giving them time to cool. As the linings heat, friction drops and the truck does not slow as it did earlier. The air pressure gauges are normal because this is not an air loss problem: the compressor is maintaining 115/113 psi, and there is no low-pressure warning. The burning smell supports the heat/fade explanation. The driver should not misread normal air pressure as meaning the brakes are healthy. Air pressure is only one part of the system; the foundation brakes can overheat and fade even when pressure is correct.
2. What should the driver do now, and what should be done at the rest area?
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Now the driver should stop using light, continuous brake pressure. Downshift to a lower gear and turn on the engine brake if conditions allow, then make a firm brake application to bring the speed down. The goal is to slow the vehicle using engine braking and short, firm snubs rather than riding the brakes. If the truck still will not slow adequately, use the rest area or the shoulder as soon as it is safe. At the rest area, park on level ground, set the parking brake, place the transmission in the lowest gear, and chock the wheels. Let the brakes cool before inspecting them; do not pour water on hot drums. Check for dragging brakes, damaged linings, and air leaks. Do not continue until the brakes have cooled and the driver is satisfied they will hold. The reasoning is that fade is a heat problem, and continuing to ride the brakes adds more heat and reduces the margin for stopping.