A few ideas - try this
1. I had the same problem with one of my cars and it turned out the actuating rod of the master cylinder was not coming back all the way when you took your foot off the brake pedal. Check the pedal, linkage, master cylinder to make sure the pedal is coming all the way back on release.
2. It sounds like your master cylinder may be leaking internally. You said you have had 3 different ones on the car. If they were rebuilt master cylinders that can sure be an issue. If the rod is coming back all the way, I would try to find a completely new master cylinder instead of the rebuilt version. I had this problem on another car of mine. I had two bad rebuilt master cylinders in a row, then went to the new one and that solved the problem.
3. Use a different method of bleeding the brakes. If your master cylinder is in the stock location, it should be higher than your wheels. If this is the case I would try gravity bleeding the brakes. I have found this is a much better method for getting rid of air in the brake system.
4. In an effort to locate the problem, bleed the system of air until you have a firm peddle. Cut a piece of lumber the right length and place it between the brake pedal and the seat so that it will maintain pressure on the system. Check the level of the master cylinder and clean all fittings, then let it set up over night. Get a good light and check everything the next day for loss of fluid, leaks, pressure on the pedal, etc.