OK, I got it. But if those hoses were damaged, wouldn't they be leaking somewhat with the level in the reservoir going down after a while? Nothing like that as far as the front lines go.. Also, the brakes were functioning perfectly well until there was a failure in the master cylinder, which probably I brought about by pressing the brake pedal all the way down when trying to bleed the system that way (by bleeding air at the nipples with an assistant who was pushing on the brake pedal). The master cylinder being very old, it likely got damaged that way.

It's a bit of a mystery to me why it still behaves this way after replacing the master cylinder and ensuring there actually is a gasket at the vacuum booster seal. I think it unlikely that the new master cylinder be also defective, which could explain what is taking place here, but I doubt it.

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1979 240D, W123, 105K miles, stick, white w/ tan interior.