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Re: Location of water tank breather pipe
The vents on my boats are at the end of 1/2" hose and the fill hose is 1 1/2". There is no check valve. They work fine with a full stream when filling. So, no, the vent hose does not have to be the same size as the fill.
If you have a check valve to keep water from overflowing your vent line when filling, then, as you fill the tank, you will trap air that becomes compressed. You will stop filling the tank when the pressure of the compressed air equals the pressure of the fill stream. This is based on observation--not conjecture--and the volume of trapped air can be significant. This is easy enough for MarkSF to verify by plugging his vent and then trying to fill the empty tank(s).
Bottom line: The vent is there to facilitate both fill and drain.
An easy fix might be extending the vent hose temporarily to drain into the galley sink when you are filling your tanks. You could coil it up the added vent hose and stow it after filling the tanks.