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I've managed to crack a piston on one of my M7100 callipers.
I have a couple of M675 ones that have been laying around doing nothing for a number of years, will they be safe to use? They were in good condition but I don't know how seals react to being dried out for years.
I put my old Boardman back to use recently, brakes are fine, haven't touched it in 5 years or so.
Maybe try lubing the pistons first.
I just resurrected some 19 year old Avid Juicys, which have hardly been used for the last 10, they worked fine
Only way to know is to try it and see. I would put the older caliper on the back so it's less of a catastrophe if it does fail.
Fit them, go for a ride, if you don't report back, we can assume the answer is no...