The check valve keeps one tank from pumping into the other. You select the rear tank that pump and sending unit is powered, as the line builds pressure fuel will follow the path of least resistance, no blockage in the line fuel flows to the other tank, a valve is directional flow control fuel in from port a can only exit port b switch tanks fuel in port d exit port b only no cross flow between tanks.
1985 F150 XLT Lariat 5.8L (351w) HO "Holly built for Motorcraft" 4bbl C6 Transmission 4X2 extended cab 8 foot bed.
"Some people spend a lifetime wondering if they've made a difference, Marines don't have that problem." President Ronald Regan 1985. I think he should have ended with: "Veterans don't have that problem." But that's me. D.A.V. USAF 1976-1992.