Vacuum develops with use.
I haven't removed the screen from my water tank vent, yet. I know it wierd, but occasionally I have to "bleed off" the vacuum in my fresh water tank by loosening the filler cap. After a shower or dishes a vacuum develops in the tank as the water is used. Obviously, this indicates a clogged vent. But, as I said earlier, when I fill my water tank and the water reaches the level of the vent hose, it will gurgle and exit the vent. It's almost like a one-way vent. Kinda strange. One time when I forgot to bleed it off and the pump began to labor as I was running the water. Soon as I bled it, it was OK.So I guess I'm talking myself into removing that one also - this weekend.