Leak-Down Test Fails on a Loaded Tractor-Trailer
A driver is pre-tripping a 3-axle tractor and a loaded 53-foot van at 0600. The outside temperature is 28°F. The governor cuts out at 122 psi and cuts in at 100 psi. With the engine off, the parking brakes released, and the system fully charged, the driver watches the gauge for one minute: it falls from 120 to 115 psi. He then holds the brake pedal fully down for one minute; the gauge falls from 118 to 112 psi. The low-pressure warning buzzer comes on at about 58 psi during a later test, and the spring brakes set at about 40 psi. The driver's route includes a 6% downgrade, four miles long, in about 90 minutes. He tells the dispatcher the tractor alone held 2 psi in one minute, so the trailer must be at fault, and he plans to top off the air and run.
1. Do the two leak-down results meet the combination-vehicle standard? Explain the arithmetic and what the tractor-only result does or does not prove.
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No. For a combination vehicle, the allowable static air-loss rate is 3 psi per minute or less with the brakes released and 4 psi per minute or less with the brakes applied. The released test lost 120 − 115 = 5 psi in one minute, which exceeds 3 psi. The applied test lost 118 − 112 = 6 psi in one minute, which exceeds 4 psi. Both fail.
The tractor-only result of 2 psi in one minute is the single-vehicle limit, so it is at the limit, not a comfortable pass, and it does not prove the trailer is the only problem. Air loss must be tested on the combination first, then isolated by capping lines or testing the tractor and trailer separately. Also, the low-pressure warning at about 58 psi is late; it should come on before pressure drops below 60 psi. The spring brakes setting at about 40 psi is within the normal 20–45 psi range. The driver should not depart on these results.
2. What should the driver check and do before moving, and why is the planned 6% downgrade a special risk?
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Before moving, the driver should treat the unit as out of service for air loss and find the leak. Likely points are the gladhands and coupler seals, trailer hoses, brake chamber diaphragms, drain valves, the trailer spring-brake chambers, and the tractor protection valve. The driver should repair or replace defective parts, recharge the system, and repeat both leak tests on the combination. He should also do a trailer brake test: charge the trailer, release the parking brake, and confirm the trailer brakes apply and release with the foot valve.
Topping off the air and running is not a fix. On a 6% downgrade, repeated brake applications demand a lot of air. A system already losing 5–6 psi per minute can drop into the low-pressure warning range, causing the tractor protection valve to shut off the trailer and the spring brakes to apply at the worst moment. That can produce trailer brake drag, loss of control, or a jackknife. If the leak cannot be corrected, the vehicle should be repaired before the trip or the load reassigned.