heres the real problem, your master cylinder is leaking into the booster from the primary cup, so you will likely need a new booster. that single master cylinder don't look right for a '66, do u have a single master on your car? go to Napa, carquest or wherever tell them what you have with all details, and tell them u need a master cylinder for it, they can get what U need. the metallic and non metallic is probably the diffrent brake linings used.