1973-1991 Dual fuel tank systems theory of operation

[hatzie]

It's real easy to swap the returns such that you're filling the un-selected tank from the engine fuel return.
I always double check the hoses on the LH side and RH side of the valve are coming from the same sender hoses. Green and Blue painters tape makes good assembly identification.

I'm not saying it's impossible to leak past the return o-rings on the valve shuttle but it would be very unusual.
Swap the return hoses on the tank side of the valve so the supply and return from each tank are on the same side LH supply & return in the LH side of the valve and RH supply and return in the RH side of the valve instead of opposite sides. This will likely fix the problem. Then you can take the replacement parts back to the store.