Automatic Slack Adjuster That Will Not Hold Adjustment
A 2015 highway tractor with automatic slack adjusters on the front and rear axles comes in with a complaint of a hot right rear drum and a pull to the right when braking. The right rear Type 30 chamber has a pushrod stroke of 2 3/4 inches (70 mm) with the brakes applied at 100 psi; the left rear is 2 1/4 inches (57 mm). The right rear automatic slack adjuster was manually adjusted twice in the last month by the driver, and the adjuster body is at about 85 degrees to the pushrod. The chamber has a clamped band, the pushrod clevis pin is worn, and the S-cam bushings show noticeable play. The drum is at its maximum wear limit, and the lining is 5/16 inch (8 mm) bonded. The truck uses a 30/30 spring brake chamber on the rear.
1. Evaluate the pushrod stroke against the Type 30 limit and explain why manual adjustment of an automatic slack adjuster can mask the real fault.
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For a standard clamp-type Type 30 chamber, the maximum pushrod stroke is 2 1/2 inches (64 mm). A long-stroke Type 30 chamber allows 3 inches (76 mm), so the chamber type must be confirmed. If this is a standard chamber, the right rear at 2 3/4 inches is over the limit and the brake is out of adjustment. The left rear at 2 1/4 inches is within the standard limit, but that does not prove it is correct. Manually adjusting an automatic slack adjuster can shorten the stroke temporarily, but it does not repair the reason the adjuster stopped ratcheting. It can also damage the adjusting clutch and hide worn S-cam bushings, a worn clevis pin, a bent pushrod, or a drum at its wear limit. Repeated manual adjustment is a warning sign that the automatic mechanism or the foundation brake is faulty.
2. Rank the likely contributing components and describe the inspection steps to separate a failed slack adjuster from a worn S-cam, chamber, or drum.
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First confirm the chamber type and the correct stroke limit. With the parking brake released and the wheels chocked, check the slack adjuster for free play, correct installed angle, and proper clevis pin fit. A worn clevis pin and elongated clevis holes add lost motion and lengthen the stroke. Next, check the S-cam bushings and cam for play; noticeable bushing play means the cam moves before it applies the shoes, increasing stroke. Measure the drum diameter against the maximum wear limit and inspect the lining; a drum at maximum and lining at 5/16 inch are both near replacement. Inspect the chamber band for cracks, the diaphragm for leaks, and the pushrod for straightness. If the slack adjuster will not ratchet when the pushrod is stroked, or if it was damaged by manual adjustment, replace it. The worn bushings and clevis pin should be repaired regardless of the adjuster.
3. What should be repaired before the vehicle returns to service, and why is the 2 1/4 inch left side not automatically acceptable just because it is shorter?
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Before the vehicle returns to service, the right rear automatic slack adjuster should be replaced if it is not maintaining adjustment, the worn clevis pin and S-cam bushings should be replaced, and the drum should be replaced because it is at maximum wear. The lining at 5/16 inch should be measured against the specification and replaced if it is near the minimum. The chamber should be checked for a cracked band, a leaking diaphragm, or a broken spring. The left side at 2 1/4 inches is shorter, but a short stroke can mean the brake is over-adjusted and dragging, or that the two sides are not balanced. Unequal stroke side-to-side can cause a pull, uneven lining wear, and poor stopping performance. After repair, set both sides to specification, burnish the brakes, and recheck pushrod stroke, leak rate, and drum temperature.