When trying to free up sticky pistons should you just clean the piston that's stuck or should you use some kind of lubricant. If it's the latter can you use, say, GT85 or better to use some brake fluid? (brake fluid to match what your brakes use obv!).

Ta….

Clean throughly with brake cleaner and a soft toothbrush. Work the piston in/out a few times. It should be fine. Rubber grease might help, but make sure all residues are removed to prevent dirt sticking to it.

Excellent, thanks Geronimo.

With Shimano brakes I would use mineral oil to lube the pistons when freeing them up.

I use cleaning alcohol to get ride of the dirt/pad residue and then use silicon spray to keep the pistons running smooth. Its works very well for my Hayes brakes.