Not repeating what you've said, merely correcting what you've said.

I won't clutter this thread up any more with acres of quotesam, but you did also say that "the manifold is always under positive pressure".
Nope, almost never...the carburetor would cease to function.
The only positive manifold pressure spikes are seen during the valve overlap period and the net result of that is a bouncy vacuum reading, rough idle, but the manifold is still under a negative pressure with respect TO THE ONLY REFERENCE POINT THAT MATTERS, which is atmospheric pressure.