I have this somewhat annoying little problem with the fresh water tank vent in my boat.
The line is clear 5/8" ID. So, when filling the tank I take the line off and whoosh.. the air comes rushing out as the tank fills.
I can blow air out the line. when it's on the tank outlet (that's not restricted either) and I fill the tank, pressure builds and makes the tank swell because the vent isn't relieving the air.
The actual vent is a good 4ft fwd and 3ft up the head bulkhead to just under the rub rail. I can only guess it's too long, but it doesn't make any sense that just air can't flow freely in the vent tube.
Apparently this problem was the cause for the poly tank to crack (we repaired it with with 3m "scotchweld" for polyethylene when I first got the boat. I don't think the PO was aware of it.