Slack Adjuster Adjustment: Automatic Adjusters and Brake Adjustment Rules

A slack adjuster connects the brake chamber's pushrod to the brake camshaft, and it sets how far the pushrod has to travel before the lining touches the drum. As the lining wears, that travel grows. Slack adjuster adjustment is the work of keeping it where the brake needs it to be.

Manual and automatic slack adjusters

• A manual slack adjuster is adjusted by hand, on a schedule, to take up wear.
• An automatic slack adjuster takes up wear on its own each time the brakes are applied. Automatic adjusters are standard equipment on air-braked vehicles built after the date the rule states.

What the rules say

Two sections of 49 CFR Part 393 bear on brake adjustment.

49 CFR 393.53(b) requires each commercial motor vehicle with an air brake system, manufactured on or after the date the paragraph states, to meet the automatic brake adjustment system requirements of Federal Motor Vehicle Safety Standard No. 121 that applied to the vehicle when it was manufactured.

49 CFR 393.47(e) sets readjustment limits for pushrod stroke. The pushrod stroke must not be greater than the values the section specifies by brake chamber type, and for chamber types the tables do not list, a limit tied to the rating the actuator manufacturer marked on the actuator.

This page does not repeat those values. The inspection standard that applies to brake stroke at an annual inspection, and how it is measured, is on Appendix A inspection standards. The rule text itself is on eCFR: 49 CFR 393.47 and 49 CFR 393.53.

When an automatic slack adjuster is out of adjustment

An automatic slack adjuster that is out of adjustment is a symptom. The adjuster is built to keep the brake in adjustment, so a brake that has drifted usually means something is wrong with the adjuster, with its installation, or with the brake components it works on, such as worn or damaged parts at the wheel end.

Adjusting an automatic slack adjuster by hand can bring the stroke back for the moment and still leave the cause in place. The brake goes back out of adjustment, and the underlying fault stays hidden. Find the cause rather than repeating the adjustment.

How to adjust slack adjusters

How to adjust slack adjusters depends on the type: manual or automatic, and the make and model. The manufacturer's documentation gives the procedure, the tools, and the safe-work steps for that adjuster. This page does not give a procedure, because a wrong step on a brake component is a safety problem. Before any adjustment, the vehicle must be secured against rolling, and the manufacturer's documentation says how to handle stored air and spring brakes.

After any adjustment or replacement, check the brake's stroke again against the applicable limit, and look at the rest of the wheel end: brake chamber air leaks and replacement covers the chamber side, and semi truck brake inspection covers the whole system.

Log the work in Blue Collar Parts

The technician can log brake adjustment work in Blue Collar Parts and send the record to their customer, who confirms it. Learn what Blue Collar Parts is.

This page summarizes 49 CFR 393.47(e) and 393.53(b) as read on the eCFR edition dated 2026-10-13. It is informational only and does not constitute legal or regulatory advice. Consult the current eCFR for the complete text.