Howdy,
i am new to the forum. I have 15 F250. Recently installed a 60 gal diesel box tank in bed stock fuel tank now leaks from top probably breather. Any easy fix?

 

First how did you connect the addl tank to the original fuel tank and what was the reason you added the addl tank?
I ask this so we have a better idea to suggest an easy fix.

Did you provide any shut off coming from the addl tank?
Unlike gasoline operated vehicles which have a charcoal canister and vented system diesels don't have any other vent than the fuel cap or separate line at the top of the original tank .With that in mind you probably have some idea of what you can/must do.
Based on your reply , that will determine where we go from there.

 

Connected with T at fill hose provided by manufacturer with ball valve. It doesnt leak if i keep valve closed. Also doesnt leak if main tank is empty and ball valve is open. Only leaks if main tank is full or more than half s tank and ball valve is open. My other F250 did it too. Is there a fix? I am wasting fuel and staining drives

 

As long as the fuel level in the aux tank is higher than original tank and valve is open the fuel will run by gravity.and overfill the original tank.

One thing you can do is, watch the fuel gauge on the dash and when it gets low ,open the valve and allow the fuel to fill the original tank and when the gauge gets close to full, close the valve,.