||I had a similar problem only my pressure was a definate pressure release when I took off the gas cap. This was after the gas tank had been removed and replace also. I found the problem to be the "vent hose" which also comes from sending unit as do the rest of the gas lines. At any rate, when I reinstalled the tank, I pinched the "vent hose" between the top of the tank and the frame of the motorhome. This cased a pressure within the tank. When I fixed the hose, it fixed the problem. You can check this out by locating the vent hose, which should be hanging over the side of the gas tank about even with the bottom of the tank, just blow into the vent tube, if you can't it is plugged.
Hope this helps.