Ford Mechanical Selector Valve Operation

Ford Mechanical Selector Valve Operation?

Ford E150 dual tank van. I need to know precisely how the mechanical selector valve operates. My van dies while driving without warning. I am down to the mechanical selector valve.....everything else checks out. would a bad selector valve result in no gas at the injectors but instead air coming out at first and then later gas? I checked the schrader valve one day and no gas. koeo the fuel pumps to prime them and no gas at injectors. turn van off and gas is now at injectors and leaking from schrader valve. I know the schrader valve core is bad and plan to replace it today but I need the symptoms of a bad mechanical selector valve and whether it would cause what happened to my schrader valve (blowing air, no gas, and then after turned off gas appears at valve.) thanks. if you want to be specific I'm all ears