Huh. By definition, hydro rears are self adjusting. You can stick a big screwdriver in there and push in the piston a bit, but as soon as you use them they should tighten up again.
That said, what you might be experiencing is a frozen caliper. In a perfect world, the caliper slides back and forth as necessary on two pins that connect it to the mounting plate. Time (and lack of lube) causes them to stop sliding and rub on one side more than the other. In an extreme case the rotor can even get warped to one side. This has happened to me 3 times on three different bikes (you'd think i'd learn).
Take the caliper off and lube those pins/bolts. They are the ones with little rubber boots on them. Anti-seize or high temp grease is the stuff to use.