So, in driving the GoldBrick (86 Syncro Passenger Vanagon) recently,
I've encountered a new issue. When I brake, if I'm lightly braking,
the pedal slowly sinks to the floor. I don't notice it sinking if I
suddenly hit the brakes, or it I pump the brakes.

No leaks at the wheels. Fluid level doesn't seem to be falling. So
I'm thinking master cylinder. Any other diagnostics to look at to
confirm (or refute) my diagnostic? Clutch seems fine.

Note, the brakes were worked on not too distantly in the past, shop
put on Big Brake kit, and I asked them to change the fluid then too...