What does a vacuum test tell you? I'm getting 9-11 inches of Hg

[WH23G3G]

I was wondering how you tell if an intake manifold gasket is leaking? I feel bad about asking for advice on a non-Mopar, but the Datsun club I belong to similar to this kind of forum, is kinda horrible no one there offers any advice maybe because they don't know or don't really care. But I know everyone here is really good at diagnosis, so I felt confident in asking. I did a vacuum test on that 1976 1.4L 2bbl  Datsun I'm trying to sell and at first it was steady at about 12-13 inches Hg between Intake manifold leak, heat riser leak, late valve timing and late igniton timing on the pressure gauge. So I checked the timing by turning the crank pulley to line up with the # 10 on the timing cover and looked at the rotor and it was pointing to the number 1 plug. So doesn't that tell you the timing is on? I then installed new points and adjusted them and new plugs. The Dwell angle was now closer to its specified range, but the meter I have is kinda crappy, it read 40-44 degrees. Specified is 44-55. Idle is fine a 750-800. I started it back up and pushed in the gas and immediately felt a major difference, no sputtering or backfire continuously, and RPMs built up strong and no bogging out after the gas pedal is released. I was pleased. I redid the vacuum test and now it reads even lower than before at 9-10 inches Hg. Right in the zone on the gauge of leaking intake manifold, heat riser, or late valve timing. I do hear a hiss of a vacuum leak while it's running, but all new vacuum hoses have been installed and routed correctly. I did the trick of spraying brake clean down the intake runners against the head and see if it was sucked in and idled up to check for a leak, but it didn't do anything different. The manual with the pressure gauge says when it's at 9 or below it indicates a leaking intake manifold gasket. Is there anything else you'll could think I could check? Idle, timing, points, dwell, coil, plugs, carb, EGR, hoses have all been checked or replaced.

[8WHEELER]

Well I have not heard the brake clean trick, I use carb cleaner around the carb or intake
if I am looking for a vacuum leak. You should not hear a hiss unless the air cleaner is off
or that noise is your vacuum leak.

Dan

[Chryco Psycho]

 timing & dwell can have an effect on the vacuum reading , you may need toi adjust both or either of these