Father-Son 1974 Duster Project

Sorry, when I first read the post, I thought you were fighting a fluid leak.

I am most likely no help on the booster, but I recently changed the master cylinder on my old truck and did not change that gasket, actually not even sure now if it was even there.

Have you actually ever put a vacuum gauge on the motor and see what it is pulling?
Low engine vacuum will cause the booster to work like crap.