I've been having an issue for over a month now with my 2001 5.3. I happen to notice when I went to fill up yesterday my gas cap had lots of pressure. Enough pressure to make the fuel tank implode slightly too! I had an EVAP code last month. I changed out the EVAP solenoid on manifold (new AC Delco part), reset the computer and code has been gone since. So obviously it's something to do with the EVAP system right? btw the code was for a p0440 which is EVAP leak. But if I have a leak why does it seem like something is blocked? Charcoal canister? Any help appreciated. It's not undrivable, but I don't want to damage my gas tank, they're insanely expensive.

Edit: Replaced vent valve by the tank but issue still persists.