Why it's usually a bad idea to rely on manual adjustment of automatic slack adjusters

Updated Apr 29, 2021

Updated April 29, 2021, republication ahead of the May 4-6 Roadcheck inspection event, taking place nationwide at a scale house near you. 

The National Transportation Safety Board recommendations to brake equipment suppliers have long emphasized caution about manual adjustments for pushrod stroke when automatic slack adjusters [ASAs] are installed. Given those warnings, “it never ceases to amaze me that folks are still attempting to manually adjust auto-slacks after their initial installation,” said Bryan Duross of Technical Training Services, producers of interactive educational modules on air brake systems and more.

Duross was writing in response to Tom Quimby’s coverage last Fall of continued issues with brake technicians approaching problems with auto-slacks by manually adjusting for pushrod stroke. Quimby’s piece was published in Overdrive sister publicationTrucks, Parts, Service.

Duross’ company’s interactive training modules, he noted, are aimed mostly at pro instructors in the programs of community colleges, technical schools, truck and coach service outfits, CDL schools and fleets. Yet he also emphasized a quick course designed specifically for individual owner-operators and/or drivers, as the tendency to manually adjust auto-slacks extends there, too, particularly in brake-related out-of-service situations at the roadside.

As Overdrive Extra contributor and longtime former owner-operator Gary Buchs has written, “Making a brake adjustment in an auto-slack may not correct a problem if there is a bigger, underlying problem with the brake system. Here is where a properly trained, advanced technician certified to inspect and diagnose brake issues can be a valuable ally.”

“We’ve addressed the issue of not adjusting auto-slacks countless times in several of our programs,” Duross said. “Recognizing the need for vastly improved driver air brake education, particularly in the United States,” the company’s “Air Brake Interactive Quick Study” courses function well for individual operators. “The programs are fully voice-narrated and employ comprehensive and detailed visuals and technical/mechanical animations that allow drivers to actually see and hear how air brake systems actually function. They also provide detailed instructions and illustrations on how to visually inspect and dynamically test a typical commercial vehicle air brake system.”

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Within that is included a detailed animation that illustrates and explains the “design and function of the AA1 automatic brake adjuster,” a common one from Haldex, Duross said.

Two example diagrams from the module — download a one-sheet flyer detailing more about the program via this link.Two example diagrams from the module — download a one-sheet flyer detailing more about the program via this link.

The tendency to manually adjust auto-slacks is longstanding, as Quimby’s original report emphasized, and out-of-service brake violations continued to top the annual Commercial Vehicle Safety Alliance (CVSA) Roadcheck inspection blitz last year. Yet manually adjusting a slack adjuster can not only lead to more brake problems down the road, it can also lead to safety-related issues, including the most serious among them — crashes.

“Despite being standard in the industry for almost two decades, there are still veteran technicians who will regularly put a wrench on an [auto-slack] to manually adjust it,” Accuride stated in an August 2020 safety and performance report for Gunite ASAs. “Overriding the automatic adjustment method can cause premature wear on the internal components and eventually result in an early end-of-life condition. Accident investigations by the National Transportation Safety Board (NTSB) have shown worn and improperly adjusted ASA’s to be a contributing causal factor in some accidents.”

Auto-slacks were mandated for use on new tractors in 1994 and trailers in 1995. The NTSB issued a scathing report in early 2006 condemning the practice of regularly adjusting ASAs. NTSB found an ASA adjustment had led to a runaway-truck accident in Pennsylvania in 2003 that claimed the life of the driver and an 11-year-old child riding in a car that the dump truck had struck during its descent on a steep downgrade.

There’s plenty there to keep in mind, no doubt. Following NTSB’s subsequent directive to brake component manufacturers requesting a change in service literature on the topic of auto-slack adjustment, “ASAs should not be manually adjusted to correct excess brake stroke,” said Jason Kraus, senior manager of braking components, Meritor. “Doing so is a dangerous practice when a brake is only out of adjustment or over stroke limitations. Excess stroke is an indication of component malfunction that manual adjustment cannot fix. Manual adjustment or de-adjustment shortens ASA life, except Meritor Stroke Sensing ASA due to its unique pull pawl design. A manual adjustment gives drivers a false sense that everything is working correctly.”

Keith McComsey, Bendix Spicer Foundation Brake director of marketing and customer solutions, ultimately, noted auto-slacks’ name speaks for itself. “Automatic slack adjusters are just that … they should act automatically,” McComsey said. “If a technician feels the need to adjust an automatic slack adjuster, it is because there is some other issue within the drum brake system that should be investigated.”

Todd Dills contributed to this report. Read Tom Quimby’s full report in Trucks, Parts, Service via this link. Access recent reporting on air disc brakes via this link.