Mate’s older shape Deore rear caliper has got a very stuck piston. So far we have held the working piston and tried to use brake pressure to force the stuck one out, soaking offending piston area with Shimano fluid – none have worked. Any ideas? there is no piston showing to get a grip on so have been unable to shift it.
All suggestions welcome!

I often get one piston moving less than another. I suspect it’s dirt/brake dust/limescale build up. I usually pump one out and hold the other in as you’ve done. I then squirt some auto brake cleaner on the piston and Clean the piston. A very small squirt of silicon spray gets it moving freely. So first thing I’d try is a good dose of brake cleaner. Suppose IPA would do the same thing.

Can’t even get the piston to move at all so can’t clean it up!

Compressed air?

Hold the moving piston still with something then pump the lever. If it still wont move try soaking some brake fluid round it. Remove the caliper from the frame/forks first and remove the pads. If its still stuck you might need to fully disassemble the caliper and replace the seals. Pretty easy job just be careful not to spill fluid anywhere.

You can’t replace Shimano seals can you?
So if you cant free off the piston then you’ll have to get a new caliper/brake.

Dunk the caliper (sans pads) in a jar of meths for a day then try again, also use a plastic tyre lever to push the piston further in then pump it back out, repeat ad nauseum…

Careful if using compressed air, if the piston does move boy will it move 🙂

Place the calliper in hot water, the piston is Pheniolic and shouldn’t expand, the caliper is aluminum and should expand. additional heat on the fluid should also cause expansion and allow the piston to move.

When you ‘hold’ the other piston, how solidly is it held? I have made a bracket in a flattened U shape to hold one piston while allowing the other to move, so that I can get full lever pressure on the sticking one. It’s made from a 30mm length of 25mm square steel box section, cut in half lengthways.

Try heating the caliper with a hair dryer while on the bike, then applying brake pressure.