Sounds like an ethanol problem
In reference to Art Duncan’s letter in the January issue on gas draining back, I have the same problem on my 1969 Dodge Coronet, 1969 Chevy pickup, 1976 Dodge Aspen, 1980 Chevy 1-ton pickup, all with mechanical fuel pumps. The 1976 Dodge has a new fuel line, fuel pump and carburetor, but it still has the same problem. I am going to try an electric fuel pump next. I think the problem is 10% ethanol in the fuel. I first experienced this problem when ethanol was introduced in Missouri two years ago. These vehicles sit for 2-3 days and the gas drains back.
The fuel obviously should not drain back when your vehicles sit for 2 or 3 days so there is something wrong. Normally, fuel drains back when the check valves in the fuel pump leak. The fact that you are experiencing this problem in all of your vehicles, plus the fact that changing the pump in the Aspen didn’t cure it, certainly does reinforce your suspicion that the reformulated fuel may be to blame.
Short of moving to another state where ethanol is not widely used (which would likely be only a temporary “fix” anyway) you have little choice but to modify your vehicles to work properly with the new blended fuel. That means making sure all of the seals in the fuel system, including those in the fuel pump check valves, are made from material that is not harmed by ethanol.