I am assuming that you see no fluid leaks. I think a small leak that you are likely to miss would more likely give you a soft pedal but it's possible that a larger leak could be collecting somewhere you don't see. Check all your fittings. If you're unsure, wrap the suspect fittings or parts of the pipes with something that is absorbent and display the leak. I've used pieces toilet paper or paper towel in the past. You could try talcum power. I'm sure there are others.
You say your bled your lines. Did you get a pedal then? When did your pedal start going to the floor? As a reminder, you should start bleeding your brakes at the cylinder furthest from your M/C and progressively get closer to the M/C. Unless your brake system has been rerouted, the cylinder furthest from the M/C is the passenger side brake caliper.
When your pedal goes to the floor, are you using up brake fluid (i.e. is the fluid level in your reservoir going down)?
If your M/C is untouched it could require a rebuild in which case I would take it to a shop that specializes in restorations (including hydraulics, of course) and hopefully they would give you a bronze liner.
Do you still have your boosters? What shape are they in? Have they been rebuilt or replaced? If they are original, I would probably replace your M/C with a 0.70 size M/C and take the boosters out. If you've got newer boosters (eg. Lockheed), your fluid may be going into the booster in which case it (or they) will need to be rebuilt or replaced or done away with as described above.
