Total Air Loss at Highway Speed
You are driving a loaded three-axle tractor-trailer at 55 mph on I-5 near Eugene. The low-pressure warning buzzer and light activate. The gauge reads 58 psi and is falling about 3 psi per second. The service brake pedal is still firm but the truck is not slowing as expected. In about 10 seconds the gauge reaches 35 psi. The spring brakes have not yet applied. You are in the center lane. There is a narrow right shoulder 150 feet ahead and an exit ramp a quarter mile ahead. Traffic is heavy behind you. The parking brake control is the yellow diamond knob; you have not touched it. The road is level and straight. You are carrying 42,000 lb of freight.
1. What is happening, and why is the service brake pedal still firm but ineffective?
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The system is losing air faster than the compressor can replace it, so the service brakes are losing their power source. The pedal may still feel firm because you are pushing against the treadle valve, but with only 35 psi left there is not enough air to apply the brakes with normal force. The spring brakes have not applied yet because they begin to set around 45 psi and are fully set near 20 psi. The low-pressure warning is telling you that you are in the last few seconds before automatic spring-brake application.
2. What should you do in the next 10 seconds? Include whether to pull the yellow knob.
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Downshift and turn on the engine brake immediately to reduce speed without using air. Move to the right shoulder or exit ramp as soon as it is safe. Do not yank the yellow knob at 55 mph; applying the spring brakes abruptly at highway speed can lock the rear wheels and cause a skid or jackknife. Use the service brake gently while pressure remains. Let the spring brakes apply automatically as pressure falls, ideally after you have slowed and are moving straight. If you must use the parking brake, do it when the vehicle is slower and aligned. Stop as far off the travel lane as possible and set the parking brake.
3. After stopping, what must you do before the vehicle can be moved?
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Do not move the vehicle until the air system is repaired and recharged. With pressure lost, the spring brakes are applied and there may be no way to release them without building air. Chock the wheels, turn on hazard lights, and place warning triangles. Call for repair or tow. Do not cage the spring brakes or bleed them to roll; that would leave the vehicle without emergency brakes. After repair, re-test build-up, leakage, warning, and spring-brake operation before returning to the road.