my older model hfx 9s are the same, i found that taking the grub screw thing out – so that oil can esacpe, and then pushing pistons back in worked well. it lets out a bit of fluid so that the pistons can recede.
otherwise get some disc cleaner, take wheel/pads out and pull the lever gently to expose some of the piston, then spray disc cleaner so that the backs of the pistons get cleaned. rinse well to knock any cr4p off and that should help too