[Oldtruckfanatic]

I'm not sure without looking up the vapor recovery system on your truck if the fuel cap is vented but What you wrote did make me think of something. Does your truck have the fuel pump with the return on it? the reason I ask is that I once worked on a truck the po had replaced the sending unit on...it was doing a similar thing and after checking hose routing I found the return was hooked to the vapor tube and the vapor tube was hooked to the return. the vapor tube isn't supposed to be submerged in fuel so it's at the top of the tank but the return is beside the pickup tube and in the fuel all the way down to I think half a tank so when the temp went up outside the pressure in the tank forced the fuel from the return tube into the vapor canister because it was in the gas. When I rerouted the lines per the diagram in the manual the trouble went away. This may not be your problem but it's worth a look if you have the return system. Hope this helps.