One of the stranger things that I've seen happen on these cars. (or any car for that matter...)
I've experienced a MC with internal leak. In the last 20+ years put replaced several on my car. Seems to me to be the most common failure. Joe's system is acting differently.
We did back track and bleed the MC after finding a loose fitting to the junction block. The MC bled fine and actually had no air in it. (which is what I would expect from and existing MC installed on a car)
We then manually bled the rears. Took some doing but we managed to chase all the air from the rear system. Had a decent pedal. Like Joe relayed, we then attempted to bleed the fronts just for good measure. (I hadn't touched the front brakes and wanted to make sure all was good)
As soon as we cracked open the front bleeder screw, the firm pedal we had was gone. After a whole bunch of bleeding, both front, which netted foam, and rear, which seemed good, pedal was still poor. At first blush I thought the MC had soured. Doesn't make sense that the MC would fail right at that moment but stranger things have happened.
I now suspect the air trapped front system must have gotten back to the MC. I believe we may need to bench bleed the MC again and start over with the wheel cyliners.

I've never had this kind of issue bleeding the brakes on these cars. I have found that vacuum bleeding nets a tighter pedal but manual bleeding works fine if time is taken to ensure all entrapped air is chased out.

I agree with Joe on this one. I think it unlikey the MC failed. I think we need to start over and confirm the MC failed (if it has) instead of just swapping parts.