Downhill Grade with Fading Brakes
You are driving a 3-axle tractor pulling a loaded van trailer, gross combination weight about 74,000 lb, down a 6 percent grade posted for trucks. The speed limit for trucks is 35 mph. You began the descent at 35 mph in a gear that holds you near that speed, but two miles down you notice that to maintain 35 mph you have had to apply the service brakes three times in the last minute, each application longer than the last. Your air gauge reads 95 psi and has been dropping slowly. The shoulder is narrow with a guardrail; the next exit is a signposted 1.5 miles ahead. Traffic behind you is light.
1. What is happening, and what is the immediate correct action?
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The repeated, lengthening brake applications with slowly falling air pressure indicate the brakes are heating and fading, and you are using more air than the compressor is replacing. The correct immediate action is to downshift to a lower gear to let engine braking hold the speed, and stop using the service brakes continuously. Snub braking - firm application to drop about 5 mph, then release and let the brakes cool - is the technique, but here the priority is getting the transmission into a gear low enough that you are not riding the brakes at all. If you cannot hold speed in a safe gear, you must get off the grade: use the shoulder or the exit, stop, and let the brakes cool. Do not continue riding the brakes hoping the exit arrives.
2. Your air pressure continues to fall toward 60 psi. What does that change about your plan?
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A falling gauge toward 60 psi is a low-air emergency, not a cooling problem. At roughly 60 psi the low-pressure warning should already be active; below that, the spring brakes will begin to apply automatically as pressure drops further. You should stop as soon as it is safe - do not try to stretch to the exit with a brake system losing pressure. Pull to the shoulder, stop, set the parking brake, and do not move until pressure recovers to normal and you understand why it fell. Continuing down a 6 percent grade with a failing air supply risks a runaway if the spring brakes set at speed on a curve.
3. After cooling and pressure recovery, how should you descend the same grade differently?
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Start the descent slower than you think you need - well under the 35 mph truck limit - and in a gear low enough that engine braking alone holds that speed without service brake use. Check the brakes at the top by applying them firmly at low speed before committing to the grade. Use snub braking only when speed creeps above your target, and keep an eye on the air gauge so a pressure drop is caught early. If the same fade recurs, the load or grade exceeds what the equipment can safely handle at that speed, and the correct answer is to find a safer route or reduce speed further, not to repeat the descent.