As technologically marvelous as modern trucks are, according to the Commercial Vehicle Safety Alliance, almost 20 percent of trucks placed out of service at roadside inspections last year were busted for problems in the humble foundation-brake system. And over half of those were cases of misadjustment.
How is that possible, when automatic slack adjusters have been mandatory on new vehicles for a decade?
The answer is that some maintenance managers take the “automatic” in ASA too literally. ASAs need inspection and maintenance like the rest of the brake system.
ASAs can fail to keep brakes in proper adjustment for many reasons, including misinstallation, improper initial adjustment and internal contamination. There even have been scattered reports from fleet members of the Technology & Maintenance Council (TMC) of new vehicles being delivered with misinstalled/misadjusted ASAs.
Moreover, ASAs sometimes appear to be at fault because of brake maintenance problems elsewhere. Things like deformed brackets, worn cam bushings, tired return springs and lack of lubrication can conspire to prevent an ASA from keeping tight reigns on brake adjustment.
Remember, these adjusting mechanisms perform within a tight window of tolerance, which corresponds to a running clearance of .020 in. to .030 in., depending on the brand. Since any slop will inhibit proper ASA operation, brake maintenance must be done to a high standard.
Slacks themselves are a good place to start inspecting. Look for deterioration or damage to boots or seals. Even a small amount of water sucked into an adjusting mechanism, especially in winter, contaminates lubricant and causes corrosion. It can freeze on tolerance-sensitive surfaces and inhibit or prevent adjustments until the ice thaws.
Next, check the stroke. TMC’s Recommended Practice (RP) 609B, “Self-adjusting and manual brake adjuster removal, installation and maintenance,” suggests that checking the power stroke is a good way to check both adjustment and foundation brake condition.
If the power stroke exceeds the maximum allowable, you’ll need to determine whether the problem is in the slack or in another foundation brake component. A good way to find out is to first measure free stroke by using a lever to pull on the slack arm until the brake shoes contact the drum.
If free stroke is over 5/8 in., suspect a malfunctioning slack adjuster. Apply the brakes and see if the splined adjusting ring is rotating in the direction needed to remove brake slack. If there’s no adjustment, the slack’s bad.
If free stroke is between 3/8 in. and 5/8 in., the out-of-spec power stroke is being caused by a foundation brake problem, often slop that can’t be felt with a lever, but which shows up under brake application.
Worn anchor pins, rollers and bushings, as well as wallowed-out holes, or bent or cracked spiders, loose chambers and bent pushrods are all slop producers. So look for the same things as when you do any good brake job.
Pay attention to details. Even the lowly S-camshaft bushing has caused problems; fleet managers surveyed say it’s the only foundation brake component that hasn’t improved much over the years.
When an S-camshaft bushing wears excessively, the camshaft thrashes around in a sloppy bore when the brakes are applied, looking for a solid surface to rest on. It may not do it on a light brake application – where most adjusting is done – but it can show up at higher psi, accounting for an out-of-spec power stroke.
Meticulous attention to detail may not take any cost out of your maintenance program, but the result is a safer truck, no out-of-service citations and less unscheduled downtime.
