Coupling at a Brooklyn Dock: The Trailer That Didn't Rise
At a Brooklyn shipper, you back under a 53-foot van with a tandem-axle tractor. The trailer was dropped at 47 inches; your fifth wheel is set at 49 inches, so the tractor must lift the trailer about 2 inches. You hear the fifth-wheel jaws close, set the parking brake, and do a tug test in low range. The trailer does not rise, but the tractor stops firmly. The landing gear is still on the pavement. You connect the service and supply glad hands, open the shutoff valves, and watch the tractor gauge: 105 psi before connection, 98 psi after. The trailer has spring brake chambers and a red line. Before you release the tractor parking brake, you notice the trailer brakes released immediately when you pushed in the red knob, and the gauge fell 2 psi in 30 seconds with the engine off. The shipper is waiting and the dock is on a 3% slope.
1. What does the failed trailer lift tell you, and what should you verify before you trust the tug test?
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A trailer that does not rise means its weight is still on the landing gear, so the tug test may only be proving that the landing gear is holding the trailer. Do not treat a firm stop as proof of coupling. The fifth wheel may be too low, or the tractor may not have backed far enough under the trailer plate. Get out with a light and verify that the fifth wheel is fully under the trailer, that there is no gap between the fifth wheel and trailer plate, that the jaws are closed around the kingpin, and that the release handle is in. Then raise the landing gear and repeat the tug test. If the jaws are not visibly locked or the trailer still will not lift, do not pull out; reposition or adjust the fifth wheel height first.
2. Interpret the air gauge readings and state whether you may move the combination.
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The drop from 105 to 98 psi after connecting is normal charging loss, but the later reading is not acceptable. With the engine off and the parking brake released, a combination vehicle should lose no more than 3 psi in one minute. You lost 2 psi in 30 seconds, which is 4 psi per minute, so the combination fails the leak test. The trailer brakes releasing when you push in the red knob can be normal if the trailer is charged, but the pressure loss shows a leak somewhere. Do not move. Chock the wheels because the dock is on a 3% slope, then check the glad-hand seals, the supply and service lines, the trailer spring brake chambers, and the trailer reservoir drains. Repair the leak and repeat the test before releasing the tractor parking brake.