Hooked up a vacuum/fuel pressure gauge (temporarily) to my new banjo bolt from Genos, and When I first start it up, I get about 6psi and then it slowly drops to around 10-15 in/vac. Cleaned pickup screen swapped out lift pump and checked supply line. Filter has been changed a couple of times recently. Truck runs the same as it has forever....I'm stumped

 

If you have a stock lift pump 6psi is max. Mine does 4-6psi at idle, 0-3psi at WOT. Need to put my electric pump on.

 

Is it possible that you are pushing the gauge past its limits, or the gauge is malfunctioning. A simple way to find out would be to get a 2 L coke bottle put a little fuel in the file may be an inch or two then disconnect the vacuum gauge and stick the hose in the fuel. If it is actually going to vacuum it will suck up the fuel if not it will start filling. You might find a way to pinch off the hose when you remove it from the vacuum gauge. Just my two cents, Mike.