Your statements sound like you bled the calipers first, then the master cylinder. Have you bled everything in the right order? Yes occasionally parts can be bad, but master cylinders usually are ok. The things that you may need to go through 3-4 on are things like carburetors, starters and alternators.... If you truly think the master is bad, just buy a new one.
But again I strongly believe that you are not bleeding the system correctly. Before you spend any ore money go through the whole system step by step and insure you didn't miss something.
Because this is not something I typically do everyday (My specialty lies in metal work, bodywork and electronics) will some one walk him through this step by step? So I do not miss something.