[Stl-986]

Disagree with Blue on the location, but your other pictures show bigger issues.

1st - replace that check valve, it's obviously broken/been messed with. Pricey for what it is. If you are able to blow air through both sides of that check valve, it is bad and can cause problems.
2nd - Dont use that pump/gauge. It will be wrong. Get a real vacuum gauge. You WILL get wrong readings with that gauge. Again, read through my entire post, lots of good info there
3rd - Your rear plenum has the rubber on the wrong side. it should be on the other by the fuel damper
4th - You only want to T into 1 place, you have it Teed into multiple.

I would get all of your vacuum system connected correctly above all. Get everything connected the way it should be (again look in my post there are diagrams/pictures). Once you do all of that then and only then should you T into a line. You want everything still connected the way it should be, all you are doing is taping into a line with the T for a proper gauge.

With the pictures you have posted it really looks like things are just not put together correctly and you have a bad check valve. I bet that once you replace the check valve & get things properly connected & installed you wont have any issues.