[Mike_The_Grad]

If there is a fitting on the intake manifold that is capped or has a small line running to some other area of the truck use that port to hook your gauge up to. A normal reading for a 4 stroke combustion engine would be 16"-21" Hg. But a lower reading does not necessarily indicate a bad setup. Like if you have an aftermarket camshaft your engine might only idle at 10-12" Hg. But this is unlikely in your situation. If you got your gauge at China Freight like I did, the gauge has info printed right on the gauge face that suggests things that possibly could be what's going on with the engine depending on what area of the gauge the needle is pointing to. You can google "vacuum gauge reading" to get an idea of what the needle is doing while you take your reading and what it indicates based on that.