Got a problem with my back brake, The piston is pushing the pad's im not sure if its pushing them out as much as it should but i have a massive gap between the pad and disk, and therefore have no back brake i even bought another caliper to see where i went wrong, and cant see anything diffrent

 

that is a hell of a gap.. i looks like your cailper is not spaced right with the disk on the wheel.. did you forget to put the washers back in when you installed the caliper?? looks off center