Pre-Trip Measurements at the Yard
You are doing a pre-trip inspection on a 2015 Kenworth T680 tractor with a 53-foot trailer. The combination has three axles: steer, drive, and trailer. With the wheels chocked, parking brakes released, and system pressure at 100 psi, you measure drive-axle pushrod travel: left 2 3/8 inches, right 2 1/4 inches. The chambers are stamped Type 30 and are standard stroke, not long-stroke. You connect the trailer lines and hear a steady hiss at the trailer gladhand. With the engine off and brakes released, the tractor gauge drops from 100 to 94 psi in one minute. The trailer supply knob is pushed in and stays in. You have not yet measured the steer-axle chambers. The yard is flat and paved; you have shop air available.
1. Using the measurements, decide whether this combination can be driven. Show the standard you applied.
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No. A Type 30 chamber with a standard stroke has a maximum pushrod travel of 2 inches. The left drive axle is at 2 3/8 inches, which is 3/8 inch over, and the right is at 2 1/4 inches, which is 1/4 inch over. Either one is an out-of-service brake defect because the pushrod is using too much of its stroke and the brake may not apply full force. Separately, the unapplied air loss is 6 psi in one minute; the limit for a combination vehicle is 4 psi per minute. That is also an out-of-service leak. The fact that the trailer supply knob stays in and the steer axle has not yet been measured does not change the decision. The vehicle must be repaired and re-tested before it moves.
2. What causes excessive pushrod travel, and why is it wrong to just adjust the slack adjuster and go?
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Excessive pushrod travel can come from worn brake linings or drums, a misadjusted or damaged slack adjuster, a loose chamber, or a bent pushrod. Adjusting the slack adjuster may shorten the measured travel, but if the foundation brake is worn, you have not restored the reserve stroke or the lining-to-drum clearance. On an automatic slack adjuster, manual adjustment can also mask the real problem and the adjuster may return to the same position. The correct response is to inspect the chamber, pushrod, slack adjuster, linings, and drum, repair what is worn, then re-measure at 90 to 100 psi with the service brake fully applied. A brake that is only adjusted to pass a measurement is still a brake that may fade or pull on a downgrade.
3. What leak checks should be repeated after the gladhand hiss is repaired?
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Reconnect the lines, build to full pressure, shut the engine off, and repeat the unapplied leak test for one minute; the combination limit is 4 psi. Then apply and hold the service brake for one minute; the limit is 6 psi. Also confirm the low-pressure warning comes on by 60 psi and the spring brakes apply in the 20-45 psi range. Check the repaired gladhand under both supply and service pressure, because a seal can leak only when the line is coupled and loaded. If the leak rate is still over the limit, the vehicle stays out of service.