try gently heating the calliper .put it in a vice and try working the piston backwards and forwards,with a pair of slip joint plyers,or try a local garage and see if they have a high pressure airline and try blowing the piston out,but only use the last idea as a last resort,also i,ve soaked one in wd 40 for a few days as well .just got to keep trying it.hope this helps as mine did this a few times,i split the calliper and that works as well
seized caliper
in Advice on Repairing and maintaining your bike
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