Trailer Supply Line Separation
A tractor-semi is travelling at 90 km/h on a four-lane highway. The driver hears a loud bang, and the red supply-line glad hand separates from the trailer. The trailer supply valve pops out, the tractor protection valve closes, and the trailer brakes apply. The tractor air gauge stays at 105 psi, but the trailer gauge falls to zero. The combination begins to slow, and the driver needs to reach the right shoulder about 250 m ahead. Traffic is moderate, and the road is straight and dry.
1. Explain what the bang, the popped trailer supply valve, and the two gauge readings tell you. Why does the tractor still have air?
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The red line is the trailer supply/emergency line. When it separates, the trailer supply valve loses its signal and pops out, which vents the trailer emergency line and applies the trailer spring brakes. The tractor protection valve then closes to isolate the tractor's air supply from the failed trailer circuit. That is why the tractor gauge holds at 105 psi while the trailer gauge falls to zero. The bang is the glad hand separating or the line whipping. The tractor service brakes should still work because they are on a separate circuit, but the combination is now being slowed by the trailer spring brakes and any tractor braking the driver applies. The driver should not assume the trailer has normal service brakes.
2. What is the correct stopping procedure, and what should the driver not do?
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The driver should signal, keep both hands on the wheel, and steer smoothly to the 250 m shoulder. Use the tractor service brake progressively; do not stab or fan the brakes. Do not use the trailer hand valve, because the trailer has no air and its spring brakes are already applied. Do not pull the yellow parking control while moving unless the service brakes fail, because that applies the tractor spring brakes abruptly and can lock the drive wheels. After stopping, set the parking brake, chock the wheels, switch on hazard lights, and put out reflective devices. The driver should not stand between the units or try to reconnect the line in live traffic.
3. After stopping, what inspection and test sequence is required before the combination can move again, and what if the red line is damaged?
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First, secure the vehicle and inspect the red glad hand, seal, coupler, and the full length of the line for cuts, abrasion, or ice. If the line is damaged, it must be repaired or replaced; the trailer cannot be safely charged without it. Do not cage the trailer spring brakes to tow the unit except as a controlled recovery by trained personnel. If the line is intact, reconnect red to red and blue to blue, start the engine, and let the trailer charge. Watch the trailer gauge rise, listen for leaks, and confirm the trailer supply valve stays in. Then perform a trailer leak test, test the trailer service brakes and spring brakes, check that the tractor protection valve has reset, and complete a pre-trip inspection. The unit should not move until the trailer brakes release and apply correctly.