Brake Fade on the 7% Grade
You are driving a 40-foot motorcoach with 34 passengers and a full luggage bay down a Montana mountain pass. The descent is 5 miles of continuous 7% grade, posted 45 mph, with a sharp right-hand curve at the bottom and wet pavement from an afternoon shower. The coach has full air brakes, an automatic transmission with a retarder, and a GVWR of about 36,000 pounds. You entered the grade at 45 mph in drive and have been using the service brake to hold speed. Two miles down, the pedal feels soft and the coach accelerates even though you are pressing harder. You smell hot brake lining and see a brake-temperature warning on the dash. A signed pullout is 1,200 feet ahead; dispatch radios that the next terminal is waiting and asks you to keep moving if possible. Traffic is light but a tractor-trailer is 300 feet behind you.
1. What should you have done before the coach reached the crest, and why is 'drive' with light brake pressure the wrong setup for this grade?
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Before the crest, I should have tested the brakes at low speed, selected a gear that holds the coach near 25–30 mph without service brake, engaged the retarder, and set my speed before the grade. On a 5-mile 7% grade, a 36,000-pound coach converts a large amount of potential energy into heat every second. Riding the service brake keeps that heat in the drums with little cooling time; once the linings overheat and outgas, the pedal goes soft and stopping force drops. 'Drive' lets the transmission upshift and the retarder may become ineffective at higher speed, so the foundation brakes do most of the work. A correct descent uses engine and retarder braking for most of the speed control, with short, firm snub applications to stay below the posted 45 mph and well below it before the curve. I should also know where the pullouts and escape ramp are before starting down.
2. The pedal is soft and the coach is accelerating with 1,200 feet to the pullout. What is your sequence, and when would you use the escape ramp?
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I would recognize this as brake fade, not a normal pedal. I would not pump or ride the brakes. I would apply full retarder, manually downshift to a lower gear, and make a firm controlled brake application if any braking remains. I would keep steering, turn on flashers, and warn traffic. If the speed continues to climb and I cannot slow enough for the wet right-hand curve, I would use the escape ramp; a 40-foot coach at increasing speed on wet pavement is a greater risk than the ramp. If I can reach the pullout, I would pull fully off the roadway, set the parking brake, chock the wheels, and inspect. I would not set the parking brake at highway speed, because the spring brakes can lock the rear wheels. Dispatch pressure does not change the physics.
3. At the pullout, the brakes smell hot and the pedal feels normal after 20 minutes. What must you check before continuing, and what is the correct decision if no mechanic is available?
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Cooling and a normal pedal do not prove the brakes are safe. I would check for dragging brakes, wheel-seal grease or oil, cracked drums, lining thickness, pushrod travel and slack adjusters, air leaks, and whether all wheels roll freely. I would compare pushrod travel with the adjustment limit; a brake that was hot may have glazed lining or a seized slack adjuster. I would do a low-speed brake test in the pullout. If I find any defect or have any doubt, I would not continue; I would call for a mechanic or a relief vehicle. 'Normal after 20 minutes' only means the pedal feels normal; it does not restore lining friction. I would never pour water on hot drums. I would document the event and notify dispatch.