My 4th gen still runs the original 15.5gal metal tank from the factory, but with a new-in-2012 delphi pump/sender assembly.

Lately I've noticed it has gotten increasingly difficult to fill up the tank after a session. I'll see the fuel gauge (stock sender going to the IQ3 dash, so I'm seeing the actual, unbuffered reading) go down after each session, but after 30 minutes of sitting I can barely get 1/2 gallon (when it should take multiple gallons) into the tank before I completely fill up the filler tube (with the fuel sender still sitting lower than full).

I've revised the fuel venting setup multiple times since this issue has become noticeable, with the latest iteration being both the 3/8" and 5/16" teed into a straight 1/2" hose, with no improvement/change. At ECR I even some vacuum on the vent hose to see if I can get the tank to "burp" and take more fuel, to no avail. It really takes it sitting for quite a while for it to be able to take more fuel.I've resorted to having to wait a couple hours for it to cool off, then jacking up the LR corner to get any reasonable amount of fuel in.

Is there a valve or something else inside the tank that's causing this?