If you drain into clean bottle then air cannot get back into the system. Also, top up master, if it goes empty then air is back in system again and you have to do full bleed again.
So your pumping up the brakes, hold, release fluid into bottle and close before foot hits the floor?

Reason I asked about the fluid is because you won't want to mix new type with old. I believe we use Dot 3 for the old cars (if memory serves me right).

If that all checks out I'm not sure what else it could be. I had a very radical cam on my car years ago with no issues with the brakes. Maybe try remove vac line from booster and plug of vac connection and test to see how they perform without vac assist. If there is little or no change then sometime wrong in the assist side.

Back in the day I would run a long vac line to my windshield so I could monitor it while I was driving. Try it on yours and see what it's pulling when your braking while driving.