Rear pistons Help please.

[milothepeanut]

I have changed the disk and pads on the rear of my 2ltr diesel x type today.
Winding the piston back on one side was hard but it went back eventually.
The problem I have is that on one side the piston wont take up the slack between its wound back position and the new pads meaning that I have to pump the pedal once before they become effective.
I am positive that I have no air in the system and all the other brakes work.
Any ideas would be good.

[8bit]

Could just be seizing up. I had one like that on my XKR, a few months after changing the rear discs and pads that caliper seized completely and was getting extremely hot just driving short distances at low speeds, had to swap out the caliper.