wierd fuel problem

[gudsgf]

hey guys, just bought me a 77 k10 and i have this wierd problem. its my first 4x4 and first truck with dual tanks. theres a bunch of hoses disconnected at this cyndrical thing under the bed. the fuel is pulling from the left tank. but as its running it fills up the right side tank and leaks out the filler neck tube. throwing my gas away, and then i run out of gas because it never pulls fuel out of the right side tank. i have know clue where to start with this. any help would be greatly apreciated thanks.sorry for the rant.