Remove the pistons carefully not to damage seals etc, take it to yout local engineering shop and get them to heat the nipples and remove them, it doesn't matter if they break as most engineering shops have access to easi-outs, a device similar to a tap which is screwed in anti-clockwise and then screwed out cw, they should have no bother geting this out and it will save you time and hassle, expect to pay £10.00 a caliper, if you have to resort to this it is ideal to replace both pistons seals roughly around £13.00 each from Vw