[Sheepdip]

You more than likely have a gas cap that only vents into the tank, it builds pressure from fuel vapors which in turn seep into your cab. You can test this by sucking and blowing on the cap. You need one that vents both ways and a new sender gasket.

When installing gasket and the locking ring use a non sparking tool (brass drift) to make sure it goes all the way home. A little Vaseline helps the rubber donut seal properly too!