Fuel leaking from air vent

You're in the right place

 

Does the tank use fuel? Or does it not level between both tanks? I am chasing the same problem and have got it narrowed down to the tank vents are bad.. If u take the vent piece off there is a plastic tube with a check ball in it. I dont know how it works. But i assume it gets stuck somehow and stops airflow and stops pulling fuel from the tank. With the return still flowing it overfills the tank and thats when it leaks.. You could shutoff the return line and siphon out some from that tank as a tempfix

 

Just dont overfill it to where its at the top of the cap and it wont leak.. Sure a dot officer would write you up for it for leaking fuel and he wouldnt be wrong.. Maybe @ZVar can cite a regulation

 

Does it have a cat in it?

 

Dont overfill it?

Realistically with a single tank setup, if its leaking when you say, the air vent or the cap will ALWAYS be where they leak, only 2 options.

Thermal expansion on an overfilled tank as the truck runs and the return fuel ends up much hotter than initial temperature means it MUST go somewhere, the cap seal and vent line are engineered failure points for this. If they werent, the tank could rupture over time, and you dont want THAT.

read the warning on the tank. "Must only accept 95% of actual capacity in normal fill conditions"

That warning label is for exactly your use case

Addition: literal first google source
"The coefficient of expansion for diesel fuel is 0.00046 per degree Fahrenheit, or roughly 1% per every 20°F increase in temperature."

This means if you fill, temp is likely 70 degrees.

If you run truck for a few hrs in summer heat, fuel temp will spike near 150-180, thats 3-4.5% , so if overfilled 2%, the tank is 100-101.5% full. (Less burnoff, calc 1 gal/hr at idle)

As such where is the fuel Supposed to go?

Bottom line. Dont overfill

 

It will only leak on operation if there is a problem with the check valve, which there always is, BUT its a cheap fix, 20 minutes and 20$. If you are on good terms with a shop tech, just do it yourself when they hand you a part

 

I put longer hoses on the the vent fittings and put a loop in the hose at the fitting and let the hose go down past the frame to the bottom of the tank. Keeps the fuel from getting out. I always over fill and top off my tanks because it keeps more moisture out when sitting in humid climates when parked for a while.