Applied Leakage and Tractor Protection Valve Failure After a Trailer Swap
After swapping trailers at a terminal, you perform the air brake tests on a 3-axle tractor with a 2-axle van trailer. The system charges to 120 psi and the governor cuts out. With the engine off and parking brakes released, the static leak test shows a 2 psi drop in one minute. You then make a full service brake application and hold it: the gauge drops 6 psi in one minute. Next you charge the trailer to 110 psi, close the tractor protection valve, and make a service brake application. The trailer brakes apply. The low-pressure warning sounds at 58 psi, and the trailer spring brakes set at 35 psi. The trailer has a current annual inspection sticker, but the shop that did the last brake work is not at this terminal. You are 20 miles from the customer and the load is time-sensitive.
1. Which of these test results are acceptable and which are not? Explain how the vehicle type affects the limits.
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The static leak result is acceptable for a combination vehicle. A tractor-trailer is allowed a somewhat higher static loss than a single-unit vehicle; a 2 psi drop in one minute is within the usual limit of less than 3 psi per minute for a combination. The low-pressure warning at 58 psi is also acceptable because it sounds before 60 psi. The trailer spring brakes setting at 35 psi is within the normal range.
The applied leakage test fails. With a full service brake application held, a combination vehicle should lose less than 4 psi in one minute. This unit lost 6 psi in one minute, which exceeds the limit. The tractor protection valve test also fails because the trailer brakes applied after the valve was closed. Either failure is enough to keep the vehicle out of service.
2. What does it mean that the trailer brakes apply with the tractor protection valve closed?
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The tractor protection valve is supposed to isolate the trailer service circuit from the tractor when it is closed. If a service brake application on the tractor still causes the trailer brakes to apply, air is leaking past the valve or the valve is not fully closing. That defeats the purpose of the protection valve, which is to protect the trailer air supply and prevent a tractor-side failure from causing a dangerous trailer brake application or air loss.
This is not a normal test result and should not be treated as a quirk. It means the valve needs repair or replacement, and the applied leakage failure suggests there may be additional leaks in the system. The combination is not safe to dispatch until both problems are corrected.
3. What should you do before dispatch, and can the 20-mile trip be justified?
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Do not dispatch the vehicle. The 20-mile trip is not justified because the applied leakage test and the tractor protection valve test both failed. A time-sensitive load does not override a known brake defect. Notify the shop and dispatcher, document the test results, and arrange for repair or a different tractor and trailer.
If a shop is available, have the tractor protection valve and the service brake system checked. The leak may be on the tractor or trailer side, so isolating the units can help the shop, but the driver should not simply 'try it' on the road. After repairs, repeat the static, applied leakage, and tractor protection valve tests before moving the load.