Found it! It was the crappy repair that Treadquarters did last year. This makes the second time their work has failed! I'd always meant to get around to fixing this properly. Now, it seems as there is no time like the present.
When they replaced the hard steel line in the rear, they left it loose, too long, and not properly secured. The shmuck just zip-tied the hose to the differential vent tube. Lazy as hell if you ask me. So, I now have a new brake hose and will tackle it in the morning.
I suppose it was a slow leak and the sudden event of Thursday was the master cylinder finally sucking air. I thought the front of the MC was plumbed to the rear and the rear to the front, but I couldn't remember. Now I'm sure!
This is what's called poor workmanship. The picture doesn't quite do justice as to how sharp the bends are in the brake hose.
Thanks for your input and help, guys!
Kevin