Can't keep fuel equal in tanks.

[TerryB]

I recently purchased a 2000 9900ix with Cummins Signature engine. I can't keep the fuel level the same in fuel tanks. It all wants to go to the right hand tank. When I bought the truck the previous owner had the left tank disconnected, he only used the right tank because of this problem. We were talking about this problem and what he had done and I asked if he'd replaced any hoses. He said he hasn't but did have them off to blow through them. Turns out he'd been running the truck well over 1.5 million miles on just 1 tank, the truck has 1.9 million on it. He said the lower fuel range didn't bother him because he would be ready for a break anyway. So I hooked both tanks back up, and yes the fuel all wants to be in right tank(tank with gauge in it). I replaced the splitter valve on the suction and return lines, that didn't change anything. I have not replaced hoses yet. The electric fuel transfer pump failed on the engine many years ago(according to the previous owner). Instead of replacing the pump with Cummins pump, he had a Fass electric pump installed. He said he did this because he liked that the Fass pump had 2 additional fuel filters, cleaner fuel, better for engine. He said the fuel leveling problem had been a problem long before the Fass pump was installed, almost back to when it was new and International never figured it out. I don't know how accurate any of that information is. I noticed the Fass pump also has a fuel return line going to the tanks, there is just a T in the line to go to the tanks. Did the Cummins electric pump have a return going to the tank also?? So as it is right now, I have a return line coming from the engine and a return line coming from the Fass pump, not sure how the factory setup was. I also don't know if the problem is in the return or if it could be sucking more fuel out of one tank compare to the other. This pump moves a Lot of fuel. When the fuel cap is removed, you can see fuel returning from the engine and there is a large amount coming from the pump return also. The next thing I'll do is remove the return lines and run them to a bucket so I can compare side to side. I'm hoping someone's experienced this and has a quick fix. The fuel tanks are both the same size, 150 gal tanks. Thanks.