Brake Fade on a Wet 6% Grade
At 6:10 a.m. you are driving a 40-foot air-brake coach with 46 passengers down a 6% mountain grade that runs 2.5 miles. The pavement is wet and the outside temperature is 34°F. You entered the grade at 55 mph in 5th gear. Halfway down, you notice the service brake pedal feels softer and the coach has crept to 63 mph even though you have been holding steady pressure. There is a faint hot-brake smell, and your air gauge has dropped from 118 psi to 104 psi. The next runaway truck ramp is 0.4 mile ahead on the right. Traffic behind is light; a box truck is 150 feet back. Your next scheduled stop is 9 miles away. What do you do?
1. What is most likely happening, and what immediate actions should you take in the next 30 seconds?
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The soft pedal, rising speed at constant pressure, and hot smell point to brake fade from overheated foundation brakes, not merely wet brakes. Wet brakes usually pull or grab; here the bus is accelerating despite pressure. Immediate actions: stop holding steady pressure because that keeps adding heat; downshift now so the engine helps slow the bus; apply firm, short snub brakes to bring speed down in steps, then release; turn on hazard lights and check mirrors; and watch the air gauge. At 104 psi you are still above the 60 psi warning threshold, but if pressure keeps falling or the low-air warning sounds, prepare to stop. Do not fan the brakes. If speed still climbs toward 70 mph or the pedal goes to the floor, use the ramp 0.4 mile ahead rather than continue.
2. If you use the ramp, what sequence should you follow, and what would make the bus undriveable afterward?
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Before the ramp: keep steering straight, turn on hazards, and tell passengers to stay seated and brace. Enter near the center at a controlled speed if possible, and do not try to steer back onto the highway. After stopping: set the parking brake, shut down, evacuate passengers to a safe area away from traffic, and call dispatch. Treat the bus as undriveable until a mechanic inspects the brakes, drums, tires, and air system. Reasons not to continue include a pedal to the floor, low-air warning, spring brakes dragging, smoke or fire, damaged tires, or any doubt about stopping ability. Do not cool down and continue just because the schedule is tight; fade can recur on the rest of the grade.