Pushrod Travel Over Limit on a Drive Axle
At a Colorado Port of Entry on US-287, an inspector asks you to apply the brakes while he measures pushrod travel. Your 2016 Volvo VNL tractor has a 48-foot flatbed trailer. The right rear drive axle uses a Type 30 clamp-type brake chamber with a maximum allowable stroke of 2 inches. At 95 psi, the right chamber pushrod moves 2 3/8 inches; the left chamber moves 1 7/8 inches. The slack adjuster is automatic, and the pushrod angle looks correct. You had the rear brakes relined 4,000 miles ago at a reputable shop. The trailer brakes are within limits. You are 60 miles from your terminal with a 12,000 lb load and a delivery in 3 hours.
1. Is the right rear brake out of adjustment? Explain using the measurement, the chamber limit, and the left-right difference.
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Yes. The right rear Type 30 chamber has a maximum allowable stroke of 2 inches, and the pushrod moved 2 3/8 inches at 95 psi. That is 3/8 inch beyond the limit. The left side at 1 7/8 inches is within limits, so the right side is not only over the limit but also mismatched by 1/2 inch. That mismatch matters because the right brake may apply later or with less force, causing uneven braking and pulling. The right brake also has little or no reserve stroke, so it may overheat or lose effectiveness during a hard stop. The recent reline does not make the measurement acceptable; it may even point to an installation or foundation-brake problem.
2. Can you simply turn the automatic slack adjuster's adjusting nut to shorten the pushrod and continue? Explain the risk.
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No, not as a normal fix. Automatic slack adjusters are designed to adjust themselves during service brake applications. If an automatic slack adjuster is out of adjustment, that is a symptom of an underlying problem, such as a worn S-cam bushing, a loose chamber, a bent pushrod, incorrect slack adjuster installation, or a foundation-brake issue after the reline. Manually turning the adjusting nut may temporarily shorten the pushrod, but it can mask the defect and may not stay in adjustment. It can also overload the chamber or cause the brake to drag. The correct action is to have a qualified mechanic inspect the entire foundation brake and slack adjuster, repair the cause, and then re-measure pushrod travel.
3. What should you do before moving, and what could cause an automatic slack adjuster to be over-stroked shortly after a reline?
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The vehicle should be placed out of service until the right rear brake is repaired and the pushrod travel is within the 2-inch limit at 90 to 100 psi. Before moving, inspect the chamber mounting, pushrod, slack adjuster, S-cam, cam bushings, drum, and shoes on that wheel. Look for a loose chamber, a worn or missing bushing, a bent pushrod, or incorrect shoe thickness. After a reline, common causes of over-stroke include installing the wrong shoe thickness, reusing badly worn S-cam bushings, mounting the slack adjuster incorrectly, or failing to set the initial adjustment properly. Repair the cause, then recheck both sides for matching travel and perform a brake test before dispatch.