Automatic Slack Adjusters

Automatic Slack Adjusters (ASA)s represent an advancement in truck brake technology. In the days before ASAs manual slack adjusters required daily inspection and manual adjustment to maintain adequate braking on heavy trucks with air brakes. But how many drivers or shop techs actually did that daily?

Back in the day, those of us who were over the road drivers were required – without the shelter of a shop and without assistance – to crawl on our backs to get under the various axles in any kind of weather to inspect and adjust manual slack adjusters.

Today’s ASAs were created to automatically adjust the length of the brake pushrod travel to achieve the optimum clearance between the brake lining and the drum and to keep brakes in correct adjustment while in normal use. They also serve to reduce maintenance requirements by eliminating daily manual adjustment. And they reduce brake application time which shortens stopping distance.


What the Experts Say

Experts agree that ASAs must never be manually adjusted except when installing them new or replacing brake linings, drums or other brake components. The ASA should never be manually adjusted to correct excess pushrod stroke which can be an indication of an incorrect ASA installation or other brake component issues.

The National Transportation Safety Board (NTSB) warns that improperly adjusted ASAs are found to be a causal factor in some heavy truck accidents. And the Commercial Vehicle Safety Alliance (CVSA) continues to pull trucks out of service for brake issues during its annual spring blitz.

Experts also say to never attempt to repair an ASA. Always opt for replacement if the adjuster can’t keep the brakes properly adjusted. If you discover the brakes to be out of adjustment by showing excess pushrod travel, try making a few service brake applications. That should bring the brakes back into adjustment automatically. If not, look for other brake component issues before condemning the ASAs.



Dangers of Manually Adjusting

Manually adjusting ASAs can result in brake drag leading to premature lining and drum wear and brake fade due to excess heat production. These factors represent unsafe conditions and can lead to accidents.

ASAs are designed to improve vehicle safety and reduce maintenance costs. So, resist the temptation to adjust them manually to correct the pushrod stroke. Dragging brakes also increase fuel consumption which adds to operating costs for owner operators and fleets, especially when it causes costly brake repairs or worse.


Don’t Forego Regular Scheduled Maintenance

Regular maintenance for your ASAs should be carried out at manufacturer’s specified intervals or mileages for PMs. Scheduled maintenance includes lubing the ASAs, but be careful not to over grease as it can result in seal failure and contamination of internal components that can cause premature failure of the ASA. Your objective by lubing is to extend the life of your adjusters.

ASAs are now in use on all truck axles equipped with air drum brakes, including steer, drive and trailer. Yet still many technicians insist on manually adjusting them to reduce pushrod stroke when that condition really indicates a faulty or improperly installed ASA or other brake component issues. In this case misdiagnosis is truly dangerous to the motoring public.

That’s why the push is on by manufacturers, safety associations and governments to stop the practice of manually adjusting automatic slack adjusters. After all, they are automatic.


Your Takeaways

The best way for a fleet to maximize the life of its automatic slack adjusters is to perform preventive maintenance at recommended scheduled intervals and never manually adjust ASAs. These two factors are key to getting the most service out of your adjusters.

Follow us