Long Pushrod Stroke and a Seeping Wheel Seal
A 2013 Class 7 straight truck has 16.5 × 7 in S-cam drum brakes on the front axle and automatic slack adjusters. The complaint is a right-front brake pull and a left-front pushrod stroke that measures 2 1/4 in after a manual adjustment; the vehicle limit is 2 in. The left-front wheel bearing shows 0.012 in of axial end play with a dial indicator, and the seal is seeping oil onto the backing plate. With the brakes released, a pry bar shows about 0.030 in of movement at the S-cam bushing. The drum has 0.008 in of radial runout. The driver says the pull started after a wheel seal was replaced at another shop 2,000 miles ago, and the slack adjuster was replaced at that time.
1. The left pushrod stroke is out of adjustment. Is the automatic slack adjuster the most likely cause? Explain your reasoning and list the checks that support or rule it out.
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Not necessarily. An automatic slack adjuster maintains adjustment only within the limits of the foundation brake. When stroke is long shortly after an adjuster replacement, the more likely causes are worn or loose foundation components: S-cam bushings, cam and roller wear, a bent pushrod, a broken return spring, a loose backing plate, or drum runout.
The 0.030 in of S-cam bushing movement is excessive and can account for lost pushrod travel. Check pushrod free play, S-cam bushing play, roller and cam wear, return-spring condition, chamber pushrod alignment, and that the correct slack adjuster is installed and clocked properly. Also confirm the adjuster actually ratchets on the return stroke when the brake is released. Do not replace the adjuster again until the foundation brake is proven serviceable.
2. Explain how the wheel bearing end play and seal leak can affect brake adjustment and pull. What measurements would you make?
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Excess wheel bearing end play lets the hub and drum move laterally and wobble. That produces drum runout and knock-back, which pushes the shoes away from the drum and increases pushrod stroke. It can also cause pulsation and a pull. The 0.012 in measured end play is beyond typical light-duty front bearing specs (often about 0.001–0.005 in), so it must be corrected.
The oil leak compounds the problem by contaminating the shoes and drum, reducing friction on that side and causing a pull. Measure bearing end play with a dial indicator while rocking the hub, check bearing condition and adjustment, and replace the seal and contaminated shoes. Recheck drum runout after bearing play is corrected, because 0.008 in runout measured with a loose bearing is not a true reading.
3. Prioritize the repairs and explain why simply readjusting the slack adjuster is not acceptable.
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First correct the mechanical faults: repair or replace the S-cam bushings, correct the wheel bearing adjustment and replace the seal, and replace oil-contaminated shoes. Inspect the drum for scoring and recheck runout after the bearing is set. Then adjust the slack adjuster and verify pushrod stroke against the limit.
Readjusting alone only masks the cause. An automatic slack adjuster cannot compensate for excessive bushing play or a wobbling drum; the stroke will go long again, and brake performance and balance will remain poor. The pull and the long stroke share the same root causes, so both must be addressed before the vehicle is returned.