Gee your brake job sounds almost as bad as the one I just did. About the only thing I didn't replace was the MC. If you have some spare MCs kicking around why not try swapping another one in? Or if you have some plugs for the master cylinder you can see if it holds pressure without swapping it. It could be air, but you sound pretty confident that you bled the brakes. The pedal usually feels "bouncy" or "spongy" when there is air in the system. Bad MC seals usually feel like there is no resistance (when they are really shot) or the pedal slowly "sinks" to the floor(leaky). But the only way to eliminate the MC is to test it with plugs, or swap it with a known good one.