Low-Air Warning, Soft Pedal, and an ABS Light
During a pre-trip inspection on a 2018 conventional bus with air brakes, you build pressure to 120 psi and hear the governor cut out. The bus is assigned to a route with a long 7% downhill grade. You drain the tanks to test the system. The low-air warning buzzer sounds at about 60 psi, and the spring brakes apply at about 45 psi. After the engine runs again, the ABS warning light stays on after the bulb check. The brake pedal feels softer than normal, and the bus has a slight pull to the right when you apply the brakes. The route starts in 25 minutes, and no spare bus is available. The outside temperature is 34°F and the road is wet.
1. Do you dispatch this bus, and what inspection steps do you complete?
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Do not dispatch until the defects are evaluated. The low-air warning at about 60 psi and spring brake application at about 45 psi are normal checkpoints, but the soft pedal, right pull, and ABS light are not normal. Complete the air-brake tests: build pressure to governor cutout, check static leakage with the engine off, check applied leakage, and confirm the warning and spring-brake application points. A soft pedal or pull can indicate an air leak, brake adjustment problem, or contaminated friction material. With a 7% grade and wet pavement, those defects are disqualifying. Notify the supervisor that no spare is available and request another bus or delay the route.
2. What does the ABS warning light mean for braking and for this route?
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The ABS light means the antilock system has detected a fault and may not modulate wheel lockup. Normal service brakes usually remain available, but the bus will not have ABS protection on a wet, downhill grade. Do not pump the brakes to try to fix the light, and do not treat ABS as a substitute for safe speed and following distance. Even if the ABS light alone might not sideline a bus in every jurisdiction, it combines here with a soft pedal and pull, so the bus should be inspected before carrying students.
3. If the bus is later cleared and you must descend the grade, how do you brake?
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Select a lower gear before the grade and keep the speed low enough that you rarely need the service brake. Use snub braking: apply the brakes firmly to reduce speed, then release them to let the brakes cool. Do not ride the brakes continuously or fan them. Maintain a large following distance, check mirrors, and use hazard lights if you are moving slowly. If the pedal fades or the bus pulls harder, pull over at the first safe place and stop; do not continue down the grade.