Safety & Compliance

Commentary: The Powder Keg of Brake Slack Adjustment

May 2015, - Editorial

by Rolf Lockwood

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Rolf Lockwood, Executive Contributing Editor
Rolf Lockwood, Executive Contributing Editor

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.


  1. 1. João Reis Simões [ May 19, 2015 @ 03:54AM ]

    As an expert in maintenance I'm atonished with what you and some experts say. I think that managers should know more about vehicles and maintenance and rely on skilled mechanical engineers. Do they exist in the Operators?

  2. 2. Frank DeMase [ May 19, 2015 @ 04:22AM ]

    Best article on brakes and safety I read in a long time.

  3. 3. Ernie [ May 19, 2015 @ 05:44AM ]

    This article reads like nothing but a bitch & moan - all complaints with no solutions or answers - Wrong - Wrong - Wrong - Directly from Bendix instructions - Manually adjust the brakes. Note: The vehicle brakes should be adjusted using the vehicle or brake manufacturer's recommendation. If they are not available, the following procedure can be used: Rotate the manual adjustment hex clockwise until the linings are snug against the drum. Turn the adjustment hex counterclockwise 1/2 turn. Pull the actuator push rod to confirm that approximately 1/2 inch of push rod free stroke exists. Apply 85 psi and check that the push rod stroke is below the readjustment limit. If the stroke exceeds the readjustment limit, check the condition of the foundation brake. Refer to Brake Maintenance Inspection. - exactly how you would field adjust a manual slack adjuster -

  4. 4. Bobby Quezada [ May 19, 2015 @ 07:58AM ]

    Ernies got it right, to correct the problem you have educate not just point out the blatant ignorance.

  5. 5. Rolf Lockwood [ May 19, 2015 @ 11:55AM ]

    Sorry, Ernie (and Bobby), that's not correct. Check out Bendix Technical Bulletin No: TCH-005-014 at

    It's pretty clear that an ASA cannot be manually adjusted.

  6. 6. Matt Hauser [ May 20, 2015 @ 09:32AM ]

    What was the percentage of fleet drivers versus owner/operators?

  7. 7. Dixon Davies [ May 23, 2015 @ 07:57AM ]

    These issues as well as other brake issues, over stroking, are why we conduct brake training to all the customers who ask for training and who purchase our products. Brake adjustment is the most critical process for safe longer lasting brakes. The ASA is only one component of the brake system that needs fine adjustment. Comments before me are all good as well as the article. Bottom line is more education and better products will provide safer brakes!

  8. 8. james alford [ May 23, 2015 @ 06:53PM ]

    Yes you can adjust automatic slacks, but the procedure as to how varies, depending on the manufacturer. How else do you think they get adjusted when you put them on. But as the article says, you can't just put a wrench on them and turn. Also as it says, if their out of adjustment, they are either bad or you have other problems

  9. 9. John Campo [ May 29, 2015 @ 06:34AM ]

    This is an excellent article and right on target. In the field of brake maintenance there seems to be a cloud of mystery surrounding the ASA and adjustment in general. For example,I would be willing to bet that the vast majority of heavy duty technicians could not tell you the difference between a stroke-sensing and clearance-sensing adjuster.
    I once had an experienced technician in a major transit bus garage tell me "Automatic slacks are not really automatic. We have to adjust them all the time."
    HOUSTON WE HAVE A PROBLEM! There is no surprise that this same fleet suffers terrible brake life and a large number of their units are found to be over-stroking.


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