Here is a photo of our puke tank, it is vented thru the alum tube welded into the bottom and inserted to the top, with the tube flush cut at the top, not contured to the top, that is how it vents thru the bottom. We have never had any oil leaking out as long as we empty the tank after every run. As you can see we also vent the water overflow into the tank, by using a fuel hi-speed can, so we can adjust the water pressure we want , mostly 5 lbs.
I hope my description is clear enough, if not call me 805-444-4489