Another bad crossover valve.

[Unilevers]

any kind of kink or restriction or too sharp a bend causes more restriction and returns more to the other tank. make sure there isnt any of those. what i did because i got tired of dealing with this on one problem truck is i just had bungs put in the bottom of the tanks and ran a hose across from both tanks to manually level them out it returned more fuel to one than the other. Many trucks dont have this issue but it seems that once you do it plagues you.