I have no issues driving just idling around the drive way, slow stop and turn, when idling in gear about 7=9 vacume. Going down the road and even in town redlights causes no problems.
Vacuum should be highest at low RPM if I recall correctly. Why is your vacuum so low? They say 18" is really required for most applications for power brakes. Some will work well in the 12" to 13" if everything is just right.
