Gas tank switch over valve stumped

[Ed ke6bnl]

Well I thought I put some foreign material in the lft tank not sure, soon after the truck would run out of gas on the lft tank and NOT switch to the rt tank with the electric switch. right tank would run fine and took me home from vacation . Pulled the left tank to check for foreign material it was completely clean and no rust. blew through all the lines and free. put tank back. still runs out of gas on the left tank and then will not switch over to the rt tank, take the supply line off at the inline clear filter b4 the carb fuel pump and then the switch over will go to the rt (good side). changed out the switch over valve still have the same problem. blew out all the lines and all seem free I AM STUMPED ON WHAT TO DO NEXT. tried to test all this without the gas caps on vent is free and this is the 6 port switch over valve with a supply return to the carb pump on my 80 dually 1ton pu. 454. and a supply return to each side saddle stock tanks. any suggestions I am lost for what to do. next. thanks Ed ke6bnl left side is energized with 12volt pwer is working to the valve.