Odd master cylinder/braking issues

This is a first for me. The car sat for a few weeks due to having stupid hot weather. Got in it to take a cruise and the brake pedal felt a little soft. Not mushy really but just a little soft. It stopped fine, didn’t feel any different in braking power or effort so I just wrote it off to not being driven for a while. Wife and I took it on a trip to the coast, 160 +\- miles round trip. All went well until we got into stop and go traffic on the way home. Then it seemed to have a brake drag. Could only feel it at speeds under 15mph. Felt like that for the last 15 or so miles. The car sat for a week and the next weekend took it out and no issues for most of the time until I got just about home and the drag was back but this time it had a shudder with the drag. Last night I put it up on stands, pulled the driveshaft and checked the joints. Put a stethoscope to the pinion and wheel bearings and all was good there. Then I pulled the wheels and drums and found a considerable amount of brake dust on the rear passenger side. Front brakes looked great. I popped the cover off of the master and fluid poured out of the front section of the master while the rear section of the master was at least half way down. Really weird! It appears that maybe there is an internal issue with the master or maybe the proportioning valve pushing fluid into the front section of the master causing a constant pressure on the rear brakes especially after multiple pedal depressions. Has anyone seen or heard of this? Is it even possible?