[nevermind65]

You must have a pretty horrendous cam to supply such little vacuum? Are you sure there isn't another problem somewhere?

Is the check valve working correctly? Have you pulled a vacuum on the booster independently to see if it holds? Is the cam degreed properly and/or initial timing set correctly?

Personally, I would look at another booster if you are determined to have power brakes instead of hooking up a vacuum pump. I had a truck with 5" of vacuum at idle and the power brakes worked fine with 4 wheel disks. I used a small diameter booster out of a 1 ton truck. Hit the pedal too hard and you would either lock them up or go through the windshield.