I don't have the metal brake disc but a brake pad has to have a metal backing, if the new brake pads don't come with one already stuck to them, you would need to put the original metal brake pads back on.
Have you looked at the linkage for the possible cause? If the brake disc you are replacing was worn, I should think you would have had to adjust the linkage as the effectiveness diminished with age. Now you have a new pad and plug, the original tension is back and that is what is causing the brakes to rub. I have had alot of trouble setting mine up in the past so I know how frustrating it can be, stick with it though and let us know how you get on.