How To Replace Slack Adjusters

When calculated by the minute, the rates that some mobile service people charge to adjust truck brakes would make many celebrity attorneys blush. Surprisingly, though, the fees – ranging from $75 to $180 for 10 minutes of labor – apparently aren’t high enough to encourage more truckers to handle the adjustments themselves.

“Most drivers don’t want to mess with the automatic adjusters,” says Carroll Underwood, an inspector at the Grapevine weigh station near Lebec, Calif. “The job isn’t that difficult, but it scares a lot of the guys.”

Underwood, a former trucker, says he always gives drivers the opportunity to adjust their own brakes when they’re out of compliance, but he also makes sure they can handle the task. Those who cannot are shown a list of nearby service providers. “I could tell them how to do it,” he says, “but with all the liability issues I really can’t say anything.”

Fortunately, Underwood adds, these types of problems have declined in recent years. He guesses that brake adjustment now represents 10 percent to 15 percent of the tickets he writes.

His estimate is comparable with the amount generated last September during the Commercial Vehicle Safety Alliance’s Operation Air Brake. About 13.4 percent of the trucks snagged in the 16-hour national blitz had too much slack in their adjusters.

Steve Howse, technical service manager at Gunite, says any improvement in adjustment compliance can be attributed to stepped-up enforcement and better technology. “Everyone is focusing on brakes these days,” he says. “No other component receives as much attention during a normal CVSA inspection.

Also, the products are more reliable. Last year, the CVSA issued twice as many tickets for manual slack troubles as for those of automatics.” That ratio is all the more significant because manuals are far outnumbered by their automatic counterparts, which have been mandated on all air-braked trucks, trailers and buses since 1994.

Scott Corbett, director of field service and fleet sales for Haldex, agrees that brakes have become the unofficial poster child of roadside inspections, but he thinks this has prompted many truckers and trucking companies to be overly cautious and distrust their automatic slacks.

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“I can tell you that a lot of the stuff coming back to manufacturers is the result of people manually adjusting their automatic slacks too often,” he says. “The practice prematurely wears out the internal clutch of an automatic. People should start their brake inspections with a tape measure, not a wrench.”

Corbett advocates power stroke measurements – used by CVSA – as a means of determining looming brake failures. “Brake components can provide valuable information when something isn’t right,” he says. “But that’s lost if you simply crank down the [auto] slacks at each PM.”

Slack adjuster makers all say their products require just two things from truckers: regular, thorough inspections and adequate amounts of grease. The payoff, of course, is greater component longevity.

“If proper maintenance is routinely done, there is no reason that a slack adjuster shouldn’t last as long as the vehicle it’s mounted to,” says Gary Ganaway at ArvinMeritor.

The benefits of extended longevity are something to consider the next time a scale master directs you to a waiting inspection lane.

5. A well-greased 10-spline S-camshaft without the slack.


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Without difficulties, slack adjusters can be swapped in less than 30 minutes, not counting time spent chasing parts or drinking coffee. Usually, the biggest challenge is loosening the old slack’s grip on the S-camshaft. Keep a bottle of acetylene handy. Product-specific information is available from the manufacturers’ websites. Instructions are also packaged with each new adjuster. The followig information applies to drum brakes.

1. POSITION THE VEHICLE. Park your truck or trailer on a level surface and chock its wheels. Release the brakes. If you must leave the brakes applied, which would be the case with an unattached trailer, you will need to cage (or manually retract the main spring of) the air chamber attached to the slack adjuster being replaced.

2. REMOVE THE ATTACHING HARDWARE. Most slack adjusters are held on the S-camshaft by a snap-ring or a thin metal clip. Use snap-ring pliers or a screwdriver to dislodge this fastener. Then remove the clevis (or yoke) and link pins. With Haldex slacks, there is only one pin to pull (the clevis), but you will also need to remove the small nut holding a control arm to its mounted bracket.

3. REMOVE SLACK ADJUSTER. Rotate the manual-adjusting bolt counterclockwise (or clockwise with ArvinMeritor products) to back off the slack adjuster so it separates from the clevis. Use an impact wrench, because this task will destroy the internal clutch mechanism found in most automatic slacks. This damage might not be a problem, however, if you’re discarding the slack being removed. Pull the slack off the S-camshaft. Parts that have been in service a long time or inadequately greased will often need to be coaxed off with a large hammer, puller or acetylene torch.

4. INSPECT BRAKE COMPONENTS. By the time a slack adjuster fails, other components in the brake assembly are probably also worn. Inspect the foundation brakes to ensure the lining is still suitable. A lot of lining nowadays is marked to indicate when replacement is necessary. The legal limit is 1/4 inch. Also check the S-cam to make sure it is straight and that its bushings and seals are intact. Finally, examine the brake chamber for cracks, excessive corrosion, bent pushrod and egg-shaped clevis pin holes. You should also remove the quick-connect clevis, if one was attached to the failed slack, and replace it with a standard threaded type. Quick-connect clevises loosen over time, and will contribute to longer pushrod travel, which could be costly during a CVSA inspection.

5. GET THE RIGHT PARTS. Before heading out for parts, measure the length of your faulty slack and note the number of its splines (10, 28 or 37). If your vehicle has ArvinMeritor adjusters, you should also look at the position of the pawl. Some of these are made specifically for left- or right-side applications. Finally, check the brand of the slack on the opposite side of the axle. Brand matching is critical because each operates a little differently, and pairing two brands on the same axle could cause excess brake wear and, possibly, unstable brake performance. Every manufacturer recommends replacing both slacks on an axle if you’re unable to find a proper replacement.

6. Slather anti-seize compound before replacing the adjuster.

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6. MOUNT THE ADJUSTER. Thoroughly clean the S-cam splines with a wire brush and solvent, then wipe dry. Apply a healthy coat of anti-seize compound to the splines. Install any required inner washers, then follow with the new slack and its outer washer(s) and retaining hardware. Use a feeler gauge to determine the amount of end play in the slack. If it exceeds 0.06 inch, you’ll need to shim the slack with additional washers.

7. CONNECT THE LINKAGE. If you’re changing brands of adjusters or just replacing a clevis, loosen the jam nut on the air chamber’s pushrod and unscrew the old clevis. Leave the jam nut in place. (Welded clevises cannot be replaced independent of the air chamber.) Apply anti-seize compound to the pushrod threads and spin on the new clevis according to manufacturer’s guidelines. Each supplies a template that will help ensure the proper mounting angle between the adjuster and pushrod. Manually adjust the slack until its clevis and link holes align with those in the clevis. Apply anti-seize compound to the pins and install them. On Haldex adjusters, install or connect the anchor bracket and tighten the fastener.

8. ADJUST THE BRAKE. Build air pressure in the system and uncage the repaired brake if you caged it earlier. Release the brakes if they had been set. Rotate the manual adjusting bolt clockwise (or counterclockwise with ArvinMeritor products) until the brake lining is pressed firmly against the drum. Then back off the bolt one-quarter to one-half a turn, depending on the manufacturer’s recommendation. Remember to disengage the pawl on ArvinMeritor slacks. Non-Meritor slacks require 13 to 70 pounds-feet of effort to back off, and they emit a clicking sound in the process. Do not use impact tools.

9. MEASURE THE PUSHROD TRAVEL. Hold a tape measure alongside the air chamber’s pushrod and brace its end against the back of the chamber. Note the clevis pin’s location on the tape. Pull the slack to its applied position and see how far the clevis pin moved on the tape. The distance, called free stroke, should be about 1/2 inch. Next, ask a helper to apply and hold roughly 85 pounds of air pressure with the brake pedal. Again, note the clevis pin’s location on the tape. The span between released and fully applied brakes is called a power stroke, and it should measure less than 2 inches for standard (type 30) air chambers and less than 21/2 inches for long-stroke chambers, which can be identified by their square air-hose ports and an attached triangular tag. Measure the power strokes of the other brakes.

10. LUBRICATE AND INSPECT. Grease all the adjusters and S-cam bushings and repeat this procedure during every service interval. Because of their proximity to the road, these parts are susceptible to damage from road debris and contaminants. Regularly inspect the brake assemblies of all wheel ends and measure their power strokes.

Power stroke measurements are more accurate (for legal purposes) than free-stroke measurements. In fact, they’re the only type the authorities use in roadside inspections.