The self adjuster has a small spring that contact the grooves in the head of the adjuster . You lift that spring and then turn the adjuster . Really, just take out the little screw that holds the adjuster in place. Then remove the adjuster. It may be stuck or may be free but don't ruin the head as new ones cost as much as the tractor, well almost. Make sure it screws in nice and free both ways, then screw it all the way in. Install the little spring. It has to allow that adjuster to turn only one direction and hold it from turning back. It may take some tinkering as it can move slightly in the screw holding it.
Those heavy springs, four of them , are what push the brake back releasing it so they have to be I good shape. As the brake is applied the head of those adjuster hit the boss inside and turn that adjuster. The other end of adjuster will limit how far it goes to maintain some clearance between discs when released. Not exactly the way everything works but you should get the idea.