How do free a seized caliper piston ?

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I am fitting new pads ,but one of the rear caliper pistons will not push back .Whats the easiest way to free it ?

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Ah yes that did cross my mind (but then I thought I was being stupid)The piston is off a Ford Sierra and on my TVR.It has a smooth face with 4 "v" shaped grooves in it .Would that be a screw back jobby ?

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I have a best mate who is a mechanic but as he was entertaining his new lady friend I didn't want to spoil his fun by phoning him . I did basicly know what I was doing but just wanted a second opinion.

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Thanks for advice all,once it was determined they are wind backs,I used a grinder disc removing tool to wind the pistons back .