i've just brought a set of 2007 XTR disk brakes [second-hand], all is good except one of the front pistons doesn't return fully. i've tried prizing them apart fully which is ok, but, when i pull the lever one of the pistons stays a little further in than it should, so if i set the capiler up perfectly with the rota in the middle it mildly rubs on one side.
i could live with this but you can feel it when braking; the sticking side pad is mildly pushing the rota over and the feel is not perfect, like most other brakes you have a lovely feeling of both pads slaping on the rota at the same time. this brake one side makes contact then the other.
i've thought about put a small amount of lube on the sticking piston but i don't what to damage the seals in any way.
any idears on how to get both pistons working prefectly in time together?