You can find your answers on pages 98-100 of this Meritor document. The adjustment proceedure they give is intended primarily for pre-setting a newly installed auto slack agjuster, and for confirming it is within tolerance. As they state on pg 99, you should never need to manually adjust a properly working unit.
"1. What is a "properly adjusted system?" How does one know that one (or more) of the automatic slack adjusters are not working properly?
2. The air brake manual states that the solution for automatic slack adjusters that are not properly adjusted is replacement of the slack adjuster. If this is true, why would anyone adjust them?"
1) since its based on maintaining the Mfg pre-detirmined gap between the brake lining and brake pad, you will notice a longer brake pedal movement, if its not automatically reducing that gap as needed. Alternately, you can measure the chamber travel as described by Meritor, and confirm its within tolerance. (this distance varies by type and size of brake chamber; and we have different chambers front/rear)
2) if the auto slack adjuster is not functioning properly, Meritor recommends a complete inspection of the entire brake system. it should not require manual adjustments.
you can see its a fairly simple mechanical device, when the brake chamber travelgap increases enough the push rod actuator engages the worm gear on release and causes the adjuster gear to rotate,decreasing the gap. As long as you keep that gear greased it should never be an issue.