Not to worry - most if not all valves leak to some degree. I consider that a nice safety feature.
More than once, In my haste to get sailing I forgot to open the valve - when I finally remembered and checked the vent, the tank was full.
That does not mean it's safe to sail with the tank empty, it just means that a leaky valve has some good features.
Conversely I've done the opposite - forgot to close the valve in my haste to get sailing - and the boat did fine - actually did not notice it was open until after a day of sailing I pulled up to the ramp, went to open the valve...well you get the idea.
Morale: haste makes waste and thanks go to the designer who understood some of us have regular senior moments and have to be protected from ourselves.
Want to know more about water ballast? Go to this link:
http://h260.com/water_ballast/water_ballast_index.html