Help Dual Tanks 84 gmc

[JustOneTruck]

Hi all Im scratching my head on this thing .

Here goes I have a 84 gmc 6.2 l Diesel with dual tanks. I put 10 gallons in and let it sit during the day I get a spot about 18 inches on the ground .I check and see and I have those dang tank protectors around it . I look but cant see anything. For some reason I open the lid on the tank and open the cap it has a ton of pressure in it. At first I think its becuase the temp of the gas in the ground is cold and it exspands at the temp of what is above ground so I put the cap back on and thinking nothing of it . The next day I go to work and park in another dry spot And after work I get the same thing. This time I pull the cap and lots of pressure again and a leak under the truck . Lastly I leave the cap loose all week and it dont leak . Not a drop . What should I be looking for. I filled it full last night no leaks this moring cap loose no drops at all

. Its 84 gmc diesel long bed --- Electric swtich for tank switching.

Thanks all