The issue has been resolved. As the master cylinder appeared to be okay, and there was no evidence of any external leaks, I tried a third bleeding. This time I used the gravity method. It took a while, but it fixed the problem. I saw some air bubbles come out of the right rear when I opened the bleeder valve, but I was working on other things as most of the bleeding was occurring, so I have not idea how much air ended up coming out.

I'm back to a firm and steady pedal.