1973-1991 Dual fuel tank systems theory of operation

[Dleslie212]

I went ahead and replaced the six port valve today, just to make sure it wasn't a problem with the valve itself. As I was connecting the six lines, I double checked the paperwork/diagram and the routing of the lines to make sure everything was in the correct spot.

Right now, both my tanks are fairly full. I filled them a few weeks ago, drove maybe 20 miles, and since then has been just idling in the driveway a total of maybe 20 minutes - so both the tanks are still pretty full. I don't have a fuel gauge hooked up just yet.

When running on one tank, if given long enough, the opposite tank starts spewing gas, as if the returns were backwards but they're not. If I switch tanks when the spewing begins, it will stop spewing after a second, and after awhile start spewing from the opposite tank

The strange part is that, before I swapped the valve today, the old valve and lines had been in place for almost three years with no problems, even with both tanks completely full. Nothing ha changed between now and then except for a complete rewire of the truck, selector switch included. Tanks, senders, straps, fuel lines and regulator were all replaced at the same time about three years ago.

Anyone have any ideas?