if there is room try using a carpenters G cramp I did with my Modeo disc cylinders and it worked a treat had em both done in 1 hour. Place a pad of cardboard between the piston and G cramp to protect the piston head. Then gently screw the cramp closing it on the piston head.You could ease the pressure by slackening the filling resevoir cap this won't let air in but may push some fluid out so check level of resevoir when job done. DON'T FORGET TO RE-TIGHTEN CAP WHEN YOU'VE DONE. If the/a G cramp head is to big for the piston the cut a piece of dowel / block of square wood that will fit the brake piston head and place that between the G cramp screw head and the piston head.
bob whiting, January 2005