I checked the piston in the prop. valve, its OK. I use the pin tool to keep it centered when I bleed them. And I know from past experience the brake light will come on when it gets off center and I don't have that currently happening any more.
I hadn't considered that the 1-1/8" bore master would take more pressure than the 1". But I do have a spare 1" I could try to see if it helps. Yes I am always bench bleeding the masters.
I went with the 9" booster for valve cover clearance.
Thinking about it more, if vacuum was my problem, shouldn't I be able to rev the engine while in park to a higher RPM and generate more vacuum for a few seconds, at least enough to "charge" the booster, then pump the brake once and I should have power assist?
This kit has the later 70's style "combination" valve, so no external block or hold off. I replaced the rear cylinders a few years ago but maybe I do have a bad or stuck one now. I'll check that also.
I haven't checked with MBM yet but when I called them on a different question a few months back and asked about that 18" vacuum requirement, they were pretty firm on that number. But at the time I didn't take it too seriously as I had heard others running much lower than that.
