redfishtd

I am really busy now don't have too much time for research but I would think that the gas tank should not have negative pressure unless the pcm is calling for integrity test of fuel system . It does this by energizing solenoid valve(Normally open valve) on carbon cannister on rear of gas tank then it opens purge valve on firewall which then less vacuum enter tank .
In your case if solenoid valve or carbon cannister is plugged and purge valve is leaking thru it would cause your problem. Purge valves are bad about leaking thru. You should not be able to blow thru them .
Topping off your gas tank can ruin the carbon cannister by flooding it.
A bad breathing thru solenoid vent circuit can cause tank filling problems ,air must exit for gas to go in making it hard to fill shutting off fuel pump due to back pressure . You can have mud/ insects blocking vent system on your tank.
If you leave gas cap off does tank show negative pressure?

getterdun

If the fuel pressure immediately goes to 40 or above when you turn the key to the on position then I would assume that the fuel pump and fdmi are working right. The only thing I know of to do next would be to check the fuel injectors to see if maybe one of them is leaking

laterongc

I’m not the OP, was deceiving my own truck for reference for the OP.

getterdun

• Oh. Wasn't paying enough attention.

I'm still unsure of Ops fuel pressure when key is first turned. But this indicates his is like yours, and if so, I would assume it's not a fuel pressure problem, and would proceed with the fuel injectors test as next step.

• Quote:

  ​​​​​​​with my truck sitting key on engine off, it does the same thing and pressure drops with time. the pic i posted of 21 psi my truck must've been sitting with key on prior.