I spent the afternoon and evening working on replacing the beetle's master brake cylinder and preparing a new rear brake circuit. I'm way too tired for a full entry, but here's the revelation of the day:
I found the leak in my brake system. It was leaking out the back of the master brake cylinder:
Note: both brake circuits have been drained and open to the air. The rubber boot on the back of the old master cylinder was completely full of brake fluid. It's not supposed to be; it's a dust cover.