Commentary: The Powder Keg of Brake Slack Adjustment

There’s an epidemic of ignorance out there regarding brake slack adjustment. Certainly not a new story, but one that we have to hammer at as often as we can. It just won’t go away, might even be getting worse.

The lousy results from last fall’s Commercial Vehicle Safety Alliance Brake Safety Week inspection blitz showed a significant increase in adjustment issues and out-of-service rates year over year.

Of the 13,305 vehicles inspected throughout the United States, Canada, and Mexico, 2,162 of them were put out of service for brake violations. The OOS rate for all brake-related violations in North America was 16.2%, compared with 13.5% in 2013.

The OOS rate for brake adjustment rose to 10.4 from 9% in 2013.

Should we be surprised? The CVSA did a survey as part of Operation Air Brake in 2003 that was pretty unsettling. It found that a shockingly low number of drivers have a clear understanding of air brakes and their adjustment. Clearly we haven’t come very far since.

How bad was it? Well, only 15 truck drivers — out of 4,055 polled — aced the survey designed to find out how much they knew about brake adjustment. The CVSA said the responses revealed an overwhelming misunderstanding about the importance of brake adjustment and the right ways to do it.

Only 192 of the drivers (0.47%) correctly identified all four of the conditions necessary to inspect adjustment properly.

More than half the drivers (62%) identified incorrect methods of determining when brakes need to be adjusted. In fact, 1,840 drivers (45%) said they rely on the “feel” of the brakes.

The auto slack myth was especially alarming in that 2003 study, which found that 2,179 drivers (53%) thought that automatic slack adjusters never go out of adjustment. While this might accurately reflect things in a well-maintained fleet, CVSA explained, it might also display a false sense of security regarding their vehicles.

A common malpractice, said CVSA, is when unqualified drivers and mechanics manually readjust auto slacks in the same way you’d take a wrench to a manual slack adjuster. But once properly installed, an automatic slack adjuster shouldn’t need manual adjustment. If it’s found to stroke beyond the maximum allowed, this pretty much always indicates other problems that need to be repaired by qualified brake service folks.

Manually adjusted auto slacks have been known to slip back out of adjustment after just a few brake applications, and confused drivers who get caught down the road can’t understand why. You can also damage an auto slack and strip the gears inside by manually adjusting it.

But the practice persists, and I trust the observations of several brake and maintenance experts I’ve spoken with recently who uniformly speak of this as a powder keg.

One veteran safety and compliance consultant tells me he’s “disturbed by the ignorance that exists amongst owners, drivers, maintenance, safety and compliance personnel...when it comes to the ‘old wisdom’ that surrounds manual slack adjusters and how they all keep trying to get that old square peg (manual slack) to fit in that new round hole (auto slack). 

“I would be curious to see the percentages of maintenance shops/mechanics that still believe putting a wrench on [an automatic] slack adjuster is the way to do things,” he told me, while wishing to remain anonymous. “I think we would find the numbers in this survey to be disturbingly high.”

I’m afraid he’s right.