Tech,
Wasn't too sure. Played with an in=dash vac guage back in the late 70s, but then it was used for 'fuel efficiency' - before the E-Shift light was commonplace.

I know 9 is too low for vac brakes, and they sell vac pumps for cars that can't, umm, get it up to 16". Now you know i couldn't pass that up!

My lincoln (and some GM cars and diesels) use 'hydroboost'.

Driven a few original 50s cars with vac wipers and such. Don't drive up hill in the rain!