Brake pedal fades to the floor?

The car listed above belonged to my father who passed 3 years ago, and it has been sitting in the driveway of my mother's house since then.
I have driven it a few miles once a month trying to keep the battery charging, oil moving, and rotors from seizing, but the disuse has clearly taken its toll.
Recently, I have been trying to put it back on the road. Have replaced all of the rear brake hardware; rotors, calipers, pads, and e-brake rigging. Have bled and bled and bled the rear lines, then the front lines as well, but the brakes now fade to the floor once I start the engine and the power booster kicks in.
Will somewhat hold pedal pressure with engine off, but once started, fades really quick. It can be recovered with a few quick pumps of the brake pedal, but just fades to the floor with a long hard press.
Thinking of the master cylinder next, but not used to VWs, and don't want to do a lot of Hail Mary replacements before somebody who knows VW better can judge on this right track/wrong track thing.
What is my real problem here?
Thanks