Well I've never had a look at the rears so I dunno if they're the same, but for the fronts: When you take the caliper bit of the carrier so it's loose, it has a part on the top and bottom kind of a slider covered in a rubber housing, it moves in and out (if you can imagine it moving towards the centre of the car and then back out). Gary sprayed WD40 under the rubber seals into the sliders and knocked either side back and forth with a hammer until they could be slided by hand. Both for the top and bottom one. Not sure whether it'll help the rear calipers, but if mine are sticky like that I'll be doing it before putting new brakes on