Man I wish I had an inspection camera to take a picture of this. I swear man, I looked at this thing a hundred times when I was wire brushing it while an 8 X 12 hole in the side of tank. Looking at it now still looks like a complete box 3/8" thick with a big 2" tall dome right on top of where that line comes in.
Oh yeah! I just did it with the KBS kit. Didn't think about blowing thru it while it was still wet. Wasn't sure tho if it had a check valve/diaphragm that only opened when a certain pressure is reached our it is just a straight pass-thru.
Strait pass-through. A metal "box" at the end of the vent line. Prevent fuel from splashing out on vent line at every movement of the bike. You need to blow air through before sealer dries. If nothing helps, clean the tank inside with MEK or Acetone (use at least 1 gal. or more) and seal it again.
BTW, sealing the tank 100% is almost impossible due to its internal shape, and construction (unless you put a huge amount of sealer like 3-4 gal. in the tank and of course drain the most of it at the end of the process).