I had that problem too when I upgraded to the 07 driller rotor. It was thicker, so I needed the piston to go in. There is little slots in the face of the piston. Not sure if there is a special tool you are supose to use for this but I just used a straight edge and screwed it back in a little ways. Make sure you have the bleeder open, so the fluid has somewhere to go.
If you still have problems, you can be an idiot like me...before I knew what was up and totally unscrew the piston and dump all the fluid out thats in there, and then scew the piston back on.....but I dont recomend that, as you wil have a ton of air in the system and you'll have the bleed the rear end.
But yeah...if you just open the bleeder and screw the piston back in you should be fine. :thumbsup: