Brake Maintenance

Keeping them adjusted makes sense for safety and compliance

May 2020

Tom Berg


Air brakes on a big rig are an obvious safety concern and therefore are one of the major items that roadside inspectors check.

They look for chafed air lines and listen for air leaks, and crawl underneath to check the condition of linings and drums on traditional S-cam brakes. And they zero in on adjustment – the amount of distance a brake chamber’s pushrod travels to move the mechanisms that press shoes against the drums. As linings wear, the travel becomes longer, and eventually linings need to be replaced. Meanwhile, even with thinning linings, adjustment must remain proper to continue to safely operate the brake at each wheel end.

Too much pushrod travel means the air chamber can’t push the shoes against the drums hard enough, and the rig’s stopping distance increases dangerously. It makes sense for owners and drivers themselves to inspect their equipment for safety’s sake and to avoid citations after an official inspection. Not so incidentally, air disc brakes are an alternative to S-cam drum brakes, and manufacturers say they have caught on with progressively run fleets and some owner-operators. That’s because air disc brakes are inherently self-adjusting and thus avoid that compliance issue.

They can also stop better than drum brakes, and they are easier and faster to maintain because pads and rotors are accessible. Many new truck models are now standard with air disc brakes, at least on steer axles.

“Brake system violations represent the most common reason commercial motor vehicles are placed out of service during roadside inspections,” says a briefing on the website of the Commercial Vehicle Safety Alliance, which is composed of enforcement agencies in the United States, Canada and Mexico. “When pushrod stroke exceeds the regulatory limit, a violation exists and something may be wrong in the foundation brake system or with the slack adjuster.”

If an officer finds a serious brake violation, a driver or mechanic must fix it before the rig is released from the inspection point or have the truck towed.

Slack defined

Slack is movement of the pushrod as air pressure forces it out of the brake chamber. Federal regulations allow 2 inches of pushrod travel for a Type 24L brake chamber and 2.5 inches for a Type 30LS (long-stroke) chamber, and the adjuster keeps that movement within safe limits. Those numbers are with parking brakes off, wheels chocked, the pedal depressed hard (by someone in the cab) and brakes fully applied. An inspector under the truck then measures the stroke. There is also “free stroke,” the movement that’s done with a pry bar by the inspector. Free stroke should be 3/8 to 5/8 inch, and if it’s more or less there’s probably a problem with the foundation brakes.

On old equipment – those built before the mid-1990s – a manual slack adjuster keeps stroke within an acceptable range. How to adjust the brakes is one of the things that drivers were expected to know (the procedure can be found in YouTube videos). Conscientious drivers would stop as they entered hilly and mountainous territory to be sure brakes were adjusted properly before proceeding down steep grades. Since the mid-1990s, air-braked trucks, tractors and trailers have been built with automatic slack adjusters, and CVSA says these have significantly reduced incidences of faulty brakes. But brakes can still go out of adjustment if something has gone wrong with either the adjuster or the brakes themselves.

Automatic adjusters can be manually adjusted but shouldn’t have to be.

If an adjuster is out of whack, either a defect has developed or there’s something wrong with the adjoining foundation brake, says CVSA. If a driver or technician corrects an automatic slack adjuster, it will probably revert to improper operation very soon. A manual adjustment is OK if the driver proceeds to a maintenance facility, where the adjuster and the brake can be properly attended to. If one is lubed, repaired or replaced, go to the opposite end of the axle and inspect the other automatic slack adjuster and its brake. And if the automatic slack adjuster is replaced, it should be with the same type of adjuster.

“The biggest mistake is mismatched equipment,” warns Jack Legler, technical director at the Technology & Maintenance Council, who says he’s been working on trucks since 1973, when he was a boy hanging around his father’s repair shop. When replacing “autoslacks,” as he calls them, “you have to go with the same manufacturer with the same series of equipment because they have to work exactly the same. If you don’t, they don’t perform the same. The amount of adjustment, amount of drag – everything is different. And if you’re working on one, you should be working on the other one on that axle to see if something is broken or just worn out. Go with like-quality, go back to the same spec, because if you don’t it will perform differently than all the other equipment on the truck.”

Members of TMC have thoroughly studied this subject and written an informational document called Recommended Practice 609C.

It reinforces Legler’s comments by pointing out that, aside from brand and series, there are four basic types of autoslacks with different dimensions and varying placements for their parts, including arm length, clevis and clevis pins, locking sleeves, and offset or straight configuration. And they require slightly different removal and installation procedures. So it’s simpler and safer to stay with the same brand and series of automatic slack adjuster. And, Legler says, don’t replace one series with a newer series because it will operate differently. If in doubt, check with the autoslack’s manufacturer to be sure you’re using the right thing.

As for preventive maintenance, every month, 8,000 miles or 300 operating hours, check pushrod travel to see if it’s within compliant specifications, RP 609C states. Every six months, 50,000 miles or 1,800 operating hours, lubricate all adjusters and clevis pins with approved oil or grease, and look for signs of wear or malfunction. If you or your fleet have more stringent requirements, more (stopping) power to you.

Cut the slack

“Slack adjuster” is the common term used by truckers and mechanics to describe the device that keeps an S-cam drum brake properly adjusted. If they work by themselves, they are “automatic slack adjusters” or “autoslacks.” But those terms are not correct, insists Robert Braswell, executive director of the Technology & Maintenance Council of ATA.

“We call them brake adjusters,” he commented. “And we don’t call them automatic. They are self-adjusting. So, it’s self-adjusting brake adjusters. I know slack adjusters is a common term, but it’s not one we use for reasons of precision.”

He’s got a point, particularly about “automatic.” When brake manufacturers introduced them at TMC meetings in the early 1990s, they emphasized that the devices worked automatically, but they did not automatically take care of themselves. They needed to be maintained, and still do.

But “slack” describes looseness in brake-actuating components, primarily air-chamber pushrods, which is why the term came into use. So we hope Mr. Braswell will cut us some slack. LL