he automatic slack adjusters reduce the amount of attention which is needed with regard to slack adjuster slack. But even automatic slack adjusters can fail to adjust the slack properly, so they still need to be checked on occasion. Automatic slack adjusters need to be greased on a regular basis to keep moisture out, and to lubricate the auto-adjust mechanism so that it keeps working properly.
Refer to the drawing below for the following explaination. As the brakes are applied, the motion of the slack adjuster causes a lifting force on the link which rotates the pinion gear clockwise. This rotation tightens the slack adjuster until the force of brake shoe contact increases the resistance on the worm gear. This increased resistance of the worm gear makes the clutch spring slip, so that the slack tightening ceases at this point. When the brakes are released, the anti-reverse spring prevents the worm shaft from turning backwards, so the clutch spring slips as the slack adjuster rotates back to the rest position. The clutch spring and the anti-reverse spring are wound in opposite directions. This is why the two springs grip and slip in opposing directions. Both springs can be overpowered by the manual adjuster hex.
Sometimes, automatic slack adjusters can over adjust, which results in brake shoes which are to tight, which causes excessive dragging of the brake shoes, which causes heat damage to the brake foundation parts. Most of the automatic slack adjusters have a manual override capability similar to the one shown above. Consult with the manufacturer for the procedures for manual adjustment if the slack adjuster sets the brake shoes to tight.