How do I isolate the leak?

The pedal shouldn't go almost to the floor. When people above say "stiff" they mean that the pedal doesn't move much when you push it from nothing to very hard. Your's sounds "spongy" or "springy". Since it doesn't continue sinking (at a constant applied force), your MC is fine. The main thing that would cause springiness is air in the lines. Explain exactly your bleeding procedure.

Before that, try backing up and stopping hard about 20 times. That will let the self-adjusters do their job on your rear drums, assuming you have them and installed them correctly. Whoever put new brake parts on my 64 Valiant reversed L & R parts so the adjusters would try to loosen the shoes each time you stopped.

"If I pump it up it'll get a little firmer". I don't understand this. The MC is not like a hydraulic pump (which has a check valve). It doesn't give you more pressure when you pump it multiple times. The only time that multiple pumps helps is if your pedal is slowly sinking to the floor, and you told us it isn't. Do some more tests and carefully note and describe the results.