Is it normal for the return fuel line to fill up a tank that isn't turned on? My front tank leaks so I have been running the back tank. Recently the front tank started leaking again, I haven't put gas in it or ran on it in two weeks. It was completely full and coming out of the lock ring on the top!

I have the tank and I am going to replace it later this week, but this doesn't sound normal to me. I was relieved to find out where all of the gas was going to. I was scared that my truck was suddenly getting 7 MPG!