National Transportation Safety Board

Facts on Truck Brake Maintenance:

Dangers of Manually Adjusting Slack Adjusters

The problem

·       Automatic slack adjusters should not be manually adjusted after the initial installation adjustment.

·       Manually adjusting an automatic slack adjuster to correct an out-of-adjustment brake will not fix the underlying problem, and the brake will likely go out of adjustment again soon.

·       When an automatic slack adjuster is out of adjustment, it indicates there is a problem with the adjuster, with the adjuster’s installation, or with related foundation brake components, which manual adjustment will not fix.

·       In a 2003 accident in Glen Rock, Pennsylvania, a dump truck with out-of-adjustment brakes was unable to stop on a steep downgrade. The truck struck four cars, and one car struck three nearby children. Two people died as a result of the accident.

·       Mechanics had repeatedly manually adjusted the dump truck’s automatic slack adjusters. They did not look for underlying problems with the adjusters or related foundation brake components, so they misdiagnosed the brake problems. They were not properly educated on automatic slack adjusters and how they relate to foundation brake systems.

·       The potential problem is enormous. According to Federal Motor Carrier Safety Administration estimates, about 8 million registered commercial motor vehicles are equipped with air brakes. Many may be operated by drivers with no air brake training, who may be poorly informed about automatic slack adjusters.

·       Even professional truck maintenance and repair organizations sometimes give out inaccurate or inadequate information on automatic slack adjusters.

What you need to know

·       The bottom line? Manually adjusting automatic slack adjusters is dangerous.

·       Don’t do it—except during installation or in an emergency to move the vehicle to a repair facility.

·       Manually adjusting automatic slack adjusters does not address the true reason why the brakes are not keeping adjustment; instead, it gives the driver a false sense of security that the problem has been fixed, though the brakes will likely go out of adjustment again soon.

·       Manual adjustment also causes abnormal wear to the internal adjusting mechanisms for most automatic slack adjusters, which may lead to adjuster failure.

·       Ask your employer for proper training.

·       Make sure maintenance and driver operating manuals are up to date and contain the correct information about automatic slack adjusters.

Need more information?

·       Visit the NTSB Web site: Highway Accident Report: Collision Between a Ford Dump Truck and Four Passenger Cars, Glen Rock, Pennsylvania, April 11, 2003 (NTSB Report Number HAR-06/01, adopted on 2/7/2006)


August 2007

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