you need to pop the pistons out too clean them properly.If thy are scored then they maybe no good.I did mine with wd40 and very fine sandpaper.One of my fronts ws sticking at this point though,4 pots 2wd.To get em out placea block of wood in the caliper and use an air line,dead easy.First time I did them my mate made a tool using a socket,pin and a nut that fitted in the piston and when you tightened it the socket expanded tus gripping the inside of the piston.