Excess fuel coming out of my front tank

The sending unit on the front tank is out, always reading zero fuel. So as a result, I always run off my rear tank. No matter what the conditions are, I almost always have fuel seeping out of my filler neck/fuel door on the front tank. To test it, I filled the front tank and drove on it for about 150 miles to run the fuel level down to about 1/2 capacity. Then I switched to my rear tank and have driven on it since then, filling the rear tank up twice since. No matter what, fuel still seeps out of the front tank filler neck. I have come to the conclusion that a) fuel from my rear tank is being pumped to the front tank or b) the return line is only feeding back into the front tank, which means I have a bad shuttle selector valve on the rear tank FDM (I think). I just can't understand how so much gas has been fed into my front tank if it is the return line to where it is overflowing. I need some professional advice please. Thank you!