ORIGINAL: Sport_Pilot
Sorry but there is no such thing as a negative pressure except relative to atmospheric. To a properly trained engineer there is no negative pressure so the manifold is always under positive pressure. Your thinking is pre space age.
There is more written about negative pressure than what 1000 "properly trained engineers" could read in a lifetime.
It doesn't even have to be relative to atmospheric to be considered negative, relative to surrounding pressure is good enough to qualify as negative.
Even I know that and I'm not properly trained.