Low Air Pressure on a Six-Percent Grade
You are driving a 72-passenger school bus with 60 students aboard down a 6% grade on Oregon Route 58. The downgrade runs about eight miles. The bus has air brakes and a properly charged system at the top: 120 psi on both gauges. You want to hold 45 mph, so you keep steady pressure on the brake pedal. After two miles the service brake pedal feels soft and the bus is harder to hold back. The low-air warning buzzer sounds and the gauge reads 62 psi. You smell hot brake lining. The next turnout is 500 feet ahead; beyond it the grade steepens and the shoulder narrows. Traffic behind includes a logging truck. Students are asking why the bus smells.
1. What are your immediate actions?
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Treat the low-air warning as an emergency. Stop at the turnout 500 feet ahead, using the brakes smoothly and downshifting to let engine compression help; do not pump the brakes or make repeated applications. Put on hazard flashers, pull as far off the road as possible, and stop. Keep the engine running so the compressor can build pressure, set the parking brake, and chock the wheels. Do not open the door and unload students on a grade with a logging truck behind. If pressure continues to fall toward the spring-brake application range, the bus may brake automatically; be ready for that and keep it straight. Notify dispatch immediately. Do not continue down the grade hoping the pressure will recover.
2. Explain why the pressure fell and the brakes faded, and describe the descent technique that should have been used.
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Continuous light brake pressure on a long downgrade overheats the drums and linings, causing brake fade-the pedal feels soft and stopping power drops. It also uses air faster than the compressor can replace it, so pressure falls. The correct technique is to select a low gear at the top and use snub braking: apply the brakes firmly to reduce speed about 5–10 mph, release them, and let the drums cool between applications. Use engine braking and a retarder if equipped. Never ride the brakes or rely on the service brake alone to hold speed for eight miles. If the bus cannot hold a safe speed in a low gear, the grade is too steep for the load and you must not start down.
3. After stopping, what checks must you make before deciding whether to continue?
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After stopping, let the brakes cool before inspecting. Check for air leaks, compressor recovery, and whether pressure builds to governor cut-out and holds with the engine off and brakes released. Look for dragging brakes, damaged drums, and fluid or grease on the linings. Do not pour water on hot brakes. If the pedal is still soft, pressure will not hold, or the smell of hot lining persists, do not continue; call for a replacement bus and transfer the students only at a safe, controlled location. A fade event on a 6% grade can repeat on the steeper section ahead. Document the incident and the repair before the bus returns to service.