Are you running an electric fuel pump, or a mechanical? Since you mention "...a third line coming off the S10 sender..." it sounds like it's the stock S10 tank, the stock in-tank electric pump, and a supply & return line to/from the engine? My guess is that the third line was the one to the charcoal canister, which is the tank vent to deal with tank pressure, mainly from changes in temperature. If you open your fuel cap on a nice hot summer day do you hear a "woosh" of pressure relieving?