My rear piston won't wind in !!

Servicing my brakes passengers side is all done and fine but the drives side is a pain in the ass ! I Cleaned it all up went to reassemble it all and found that I couldn't get the calliper back on. After further investigation it seems that the piston has wound out some how ! I attempted to wind it back in and it will turn but it won't move in far enough back in to get the calliper back on. I have put a lot of force behind but it just won't go ! Any ideas what the problem is or what I can do !?
 
Take the cap off the fluid pot, should take any pressure out of the lines
 
once to goes it will become easier

i used a g clamp on my 197 to keep pressure on the piston while i wound it in with some pliers

on my 200 i used the tool and it was crap but got there in the end
 
Sure your winding it the right way?

and make sure the hand brake is off
 
I did mine last week, I bought a caliper rewinder because there was no chance it was going in using pliers!! Still took quite a bit of force.
 
I did mine last week also, I found that a two prong chuck key (Picture Below) from a 115mm angle grinder lines up with the grooves in the piston pretty well. In combination with a G-clamp you can apply a good amount of force. If you have one laying around id give it a go.

untitled_zpsf8b75275.png