Front fuel tank is transferring fuel to the rear or fuel is being return to it

I have a the truck listed above lightning with dual tanks I did some upgrades to the engine and had to install high performance fuel pump after that was done my rear tank is always overflowing with fuel and my front tank goes empty pretty fast, they told me it could be my fuel tank selector valve, but my mechanic can’t find it. They told me that maybe my truck didn’t come with one.