Displacement of the magnetic body of a spring engaged, friction brake or clutch, is automatically adjusted for releasing operation by a valve controlled fluid pressure resisting stop device. Movement of a spring loaded armature in response to wear of a friction disc engaged therewith, displaces a piston confining a pressurized body of fluid by opening of a valve from which a valve rod extends to form a limit stop engaged by the magnetic body that is displaced by the armature in one direction.