Odd master cylinder/braking issues

Most likely not the problem than with manual brakes.

But it seemed that your issue has gotten progressively worse over time.

Professor Fate's auto adjuster thoughts are worth checking.

The MC could be sticking at the release end of the travel not allowing the fluid to go back into the rear resivour OR an internal leak from the front brake system to the rear brake system.
I’ll be checking that for sure. I was planning to convert over to the two bolt master this fall, maybe doing that sooner than I thought.