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When i had this problem, i washed off the crap from the calliper with isopropyl alcohol, then got one piston gently out and washed it down with more alcohol, then lubed the sides with mineral oil and let it retract back in. Repeated for the other piston and then washed it all down with alcohol with the pistons back in their retracted position.
I had thought my caliper was fucked and had even ordered a replacement but the above routine solved the problems.
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then washed it all down with alcohol
This is @hippy 's normal approach to a problem.
Do as Ed says (not often you read that on here). It sounds like you've got a sticky piston, which usually means the brake works fine, but it the pad doesn't retract properly when released. A quick clean and lube of the piston will normally rectify it.