if you have access to air line blow pistons out but not completely use a piece of wood etc to stop them so that all are nearly out otherwise one will blow out others will then not be able to ,then undo 4 bolts clamping halves together ,keep hold of seals in two halves and pull pistons out ,remove seals and clean parts thoroughly ,when refitting pressure seal (inner one)rub your finger from inside bore outwards you should feel seal lip as you do to ensure fitted correctly outer seal and retainer fitted as one using suitable dolly and tapped in square ,if you dont have access to air line remove pads and using brake pedal and suitable spacer press piston so all are nearly out before removing caliper