Thanks for the thought, I've rebuilt the master cylinder using a kit and everything seems to have gone back as it should and all the bits were the same as I removed, no leaks or other concerns. My thoughts were the same as yours initially so I went with the kit. If I couldn't achieve the right feeling with the piston back in the calliper I would strip it again and check the assembly etc... but I guess it wouldn't work by creating any pressure if it wasn't working.