Brake Fade With Normal Air Pressure
You are driving a 78,000 lb five-axle combination down a 7% grade that is 3 miles long. It is 90°F, dry, and you are in 7th gear at 35 mph. You use the foot valve to hold speed. Near the 2-mile point, the pedal is still firm and the air gauges hold at 110 psi, but speed has climbed to 52 mph despite a hard application. You smell hot brake lining and see light smoke from the trailer wheels. There is a pullout 500 feet ahead. The engine brake is on, but you have not downshifted. The low-air warning has not sounded. The trailer has drum brakes and automatic slack adjusters; the last inspection was 3 months ago.
1. Is this an air-loss problem or a brake-fade problem? What evidence supports your diagnosis?
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This is brake fade, not air loss. The air gauges are steady at 110 psi, the low-air warning has not sounded, and the pedal is still firm. Air loss would show falling pressure and likely a changing pedal feel. The smoke, hot-lining smell, and speed increase despite a hard application point to overheated drums and linings. As the drums heat and expand and the lining friction drops, the same pedal application produces less stopping force. The automatic slack adjusters may also be taking up more pushrod travel as the drums expand, but the root problem is heat, not a leak.
2. What should you do immediately, and why is continuing down the grade a bad decision?
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Downshift to get more engine braking, keep the engine brake on, and pull into the pullout 500 feet ahead. Do not continue down the grade hoping the brakes will recover; fade usually gets worse as the drums keep heating, and the next section may require more braking than the faded brakes can deliver. After stopping, set the parking brake, chock the wheels, and let the brakes cool. Do not spray water on hot drums or inspect them closely while they are hot. Continuing also risks a fire or a complete loss of braking on the trailer.
3. What descent technique should have been used, and what should be inspected before the vehicle moves again?
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Before starting the grade, select a gear low enough to hold speed with engine braking and use snub braking: apply the brakes firmly to reduce speed by about 5-10 mph, release, and repeat rather than riding the pedal continuously. After the brakes cool, inspect linings for glazing or cracks, drums for heat damage, slack adjusters and pushrods for travel, and seals and hoses for heat damage. If a drum is cracked, a lining is glazed or contaminated, or pushrod travel is beyond the limit, the vehicle should not continue until repaired.