fuel tanks not leveling

There is a valve in the T where it splits to the tanks, I think. You need to determine if the fuel is not returning to the drivers side tank or if it isn't sucking out of the passenger side tank. It could be a bad hose on the passenger side suction line(they collapse internally) or a cracked standpipe. Try taking off the return line on the driver side and put it in a jug or pail to see if it's returning fuel to the tank.

 

There must be a restriction in the passenger side making it draw harder. When both are hooked up it sucks the other side dry. Maybe the plastic or cardboard seal from a bottle of fuel additive got dropped in the tank. Could be bad fuel line from tank to the T.

 

Put a deverter valve on the return side should fix it

 

Also make sure the hoses are routed the same height and not dipping down on either side, I have a truck that sometime in it's past the hoses were changed and one side was made a foot too long and I'm figuring that the fuel took it's easiest path of resistance because it wouldn't pass the extra dip in the hose. We cut the hose and problem solved.