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

[60-66 Nut]

Did you try a new cap yet? I've seen the check valve go bad in them before.

60-66 Nut

[86 GMG]

replace cap first and then we can go from there like 60-66 nut said

[Drivin' Miss Daisy]

Might also check the rubber hose from the filler to the tank. My truck did similar thing and that is what it turned out to be. Also if you top off the tank to max full it can leak from around this hose until the level of fuel is below the hose. I just had to tighten the hose clamps and the problem was solved.

[80BIG10]

I kinda have the same problem. I had the truck in the shop last week to take care of some alignment problems and the guy comes back to me at the end and tells me I have a fuel leak (but they don't work on fuel systems). He said it was inside the passinger side frame rail where the fuel line and the fuel pump hook up. There is a small ruber (3/8's) line that conects each line to the pump. Now I did smell gas the day before I took it in but thought it was another car next to me. Needless to say I have filled both tanks - switched between the two tanks - left the truck running for 20 min each time and can find nothing and I haven't smelled anymore gas over a week now.

I guess I'm just going to wait until I smell gas and then crawl under again and see where exacly it is coming from.

[JustOneTruck]

New caps in place .I`m still getting a leak now . Looks like its time to take them tanks down and replace or replace the rotted hoses on them.
Any one have great directions on the best way to remove these dang tanks.

Thanks