Automatic, but not maintenance free

Since the purpose of automatic slack adjusters (ASA) is to maintain proper shoe-to-drum clearance in all braking modes, how reliably it performs this task can affect braking performance and brake life.

Meritor emphasizes that even though they're automatic, ASAs are not maintenance-free. Fleets must periodically check for grease, with intervals determined by vocation. Slack adjusters used in linehaul operations, for example, don't have to be checked as often as those used in frequent-braking service such as garbage trucks and school buses. Crewson Brunner recommends greasing when you do PM on the brake system.

Most manufacturers also recommend greasing slack adjusters at installation whether they're supplied loose or preassembled to the brake. Meritor points out that even though pre-assembled slack adjusters have been greased at the manufacturer, it's prudent to grease them when you take delivery of a vehicle. Exceptions are no- or low-maintenance ASAs, such as those standard on Meritor's LX500 Q Plus cam brakes.

Bendix recommends greasing slack adjusters at, every 25,000 miles or at PM, as well as at installation. The company's ASA-5 SureStroke, an unhanded unit for tractors and trailers, is designed with a fitting for the mechanic's grease gun. You know the ASA is properly lubed when clean grease flows out of the boot.

Dana Spicer, which sells ASAs made by Haldex, offers units with three levels of lubrication. The traditional ASA requires lubrication annually. A second model has a special fitting that comes with low-lube LMS brakes; lube intervals are 250,000 mi. A no-lube version is available with Spicer No-Lube LMS brakes.

After brakes have been relined, Meritor recommends adjusting the ASA manually to a certain clearance and then letting the slack adjust to proper clearance. Since the slack adjuster's function depends on having brakes fully back, it's also important to replace the shoe-return spring with a new one when a brake is relined.

Crewson Brunner cautions that making too many manual adjustments can cause ASAs to wear out prematurely. If the ASA isn't holding adjustment, then you have either a faulty unit, an incorrect installation or a problem with the foundation system. And according to Bendix, other indications that an ASA is not working properly include one side of the vehicle braking more aggressively or an angle that's greater than 90 deg. between the slack adjuster and the chamber pushrod.

Manufacturers agree that different brands of ASAs should not be used on the same axle. Since slack adjusters are designed to maintain clearance slightly differently or set up at various angles, depending on the brand, mixing brands could create an imbalance. When it comes to using different brands on different axles, opinions vary slightly. Some say it's not an issue at all; others say it's better to use the same brand, but admit that the consequences of not doing so are nowhere near as severe as they are for mixing brands on the same axle.

Haldex, which uses the term automatic brake adjuster, or ABA, introduced a self-setting option earlier this year. The S-ABA is designed to function correctly no matter what the position of the control arm. By eliminating the control-arm adjustment needed to get correct running clearance, installation of the S-ABA brake adjuster is faster and easier.

Crewson Brunner offers Auto Grease and Auto Check options on its slack adjusters. Auto Grease lets you keep the S-cam fully lubricated when you grease the ASA. "The specially designed spline gear has a groove in the center that allows grease to flow to the S-cam, promoting longer life for the S-cam and ASA," says the company. Auto Grease will become standard on Crewson Brunner slack adjusters later this fall. Auto Check gives you a visual indication of ASA status. When brakes are applied, a pointer measures working stroke and confirms that the ASA is functioning properly.

Gunite ASAs sport several design changes this year, including a precision fit, collar-lock clevis that provides tight tolerance between the push rod and clevis that reduces maintenance time by enabling technicians to dis-assemble the slack from the brake without having to take the pins out. A self-lubricating worm wheel also decreases maintenance requirements.

What does the future hold for ASAs? Although we're not likely to see much change in basic design, "smart" vehicle technology could one day expand the role of this component. Dana Spicer envisions the day when smart ASAs will provide electronic feedback to warn drivers of problems.