A slack adjuster comprises a worm shaft that is configured to drive a worm gear adapted for coupling to a camshaft. An adjustment assembly receives slack adjustment input from a push rod and is configured to automatically rotate the worm shaft when brake slack exceeds a predetermined limit. The slack adjuster includes a manual adjuster that is moveable between a non-adjustment position where the manual adjuster is de-coupled from the worm shaft and a manual adjustment position where the manual adjuster is coupled to the worm shaft such that a position of the worm shaft can be manually adjusted as needed.