Total Air Loss at a City Intersection
At 35 mph on a four-lane city street in Tulsa, Casey is driving a 26,000-lb straight truck with air brakes. The truck has a dual-circuit system. Without warning, the brake pedal goes to the floor with no resistance, and the truck does not slow. The dash gauge shows 65 psi and falling; the low-pressure buzzer sounds, and the red warning light is on. The parking brake control is a yellow diamond push-pull knob. Ahead is an intersection with a red light 300 feet away, and a shallow right-side curb with an empty parking lane. Traffic is behind him. The truck is loaded with fragile cargo. The engine is running and the transmission is in 5th gear (manual). The street is level and dry.
1. Explain what has happened in the air system and what the spring brakes will do as pressure falls.
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The service brakes have lost air pressure, so the pedal has nothing to push against. The gauge at 65 psi and falling means an active leak or compressor failure; the low-pressure warning is correctly on. The spring brakes are held off by air pressure, and they apply automatically when pressure falls into roughly the 20-45 psi range. The yellow parking brake knob does the same thing manually by exhausting air from the spring chambers. At 35 mph, a full spring-brake application can lock the rear wheels and cause a skid, but as pressure keeps falling the application will happen whether Casey is ready or not. His remaining time is measured in seconds, not minutes.
2. What action sequence should Casey use in the next few seconds?
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Keep the truck in gear and downshift progressively to use engine compression; do not shift to neutral and do not pump the pedal. Turn on hazards and sound the horn, steer toward the empty parking lane rather than the occupied intersection, and use the curb if necessary to scrub speed. Delay pulling the yellow knob until the speed is lower if he can, because a full spring-brake application at 35 mph may lock the rears. If the knob must be used, pull it and be ready to steer through a skid; releasing it may not be possible once air pressure is gone. A controlled stop with some cargo shift is better than entering the red-light intersection at speed.
3. Why is pumping the pedal useless, and what should Casey do after the truck stops?
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Pumping the pedal is useless because there is no stored air left to apply the service brakes; each pump only releases more air and can make the spring brakes set sooner. After the truck stops, he should chock the wheels, set the parking brake, shut off the engine, place warning devices, and call for help. He should not try to drive or repair the system on the street. He can check for an obvious broken line or gladhand, but the vehicle needs a mechanic. The cargo is fragile, but the priority is preventing a runaway truck from entering the intersection; the load can be inspected and secured after the stop.