I'm looking at adding a "boost controller" to tweak some vacuum levels in various places on my system. Mainly for controlling the transmission.

I'm aware I need to sort out other gremlins first, this is more of a fact-finding mission.

I want to put the boost controller on a vent line. If plugged, it's venting "0 psi"...I figure I could set it up so that it vents maybe, 5 psi or something like that.

Would it work in that manner? Or is it only designed for stuff blowing into it, versus sucking?

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That. Vacuum does not work in PSI, it works in inches of water or mercury.

The most you can likely do is a check valve or a restriction orifice.