This is a classic common problem with these trucks. I am surprised I haven't seen it on here like you see air intrusion and fuel pump threads.
Even the gas trucks can have this problem,, but the diesels have it more because the fuel foams so bad. When you are pouring all that fuel into the tank at a high rate, the same volume of air needs to get out of the tank at the same time. There is no where for this to happen except the filler hose.
The factory had a small hose inside the larger hose to try a separate the fuel going in from the air going out. But as you can see it didn't work that well. The best solution I have found was mentioned by the post above. I took out the roll-over valve in top of the tank, but left the grommet in place. I found a copper elbow fitting (I think it's 3/4) just fits inside the grommet, and using a 90 degree fitting keeps it close to the tank top. There is very little room above the rear tank.
I ran this added vent on the top of the tank with a large hose and tied it to the frame. The better option is what the poster above mentioned, putting a fitting in the fill neck. With mine I will get a little puke of foam out of my vent hose when filling the tank to the top at the station. And if you put the fitting in the filler, put it as high as possible. You need to be able to put the fuel station gun past the air vent so the air doesn't interfere with the fuel going in.
keep up the good work.
