Hi Pen, what you've described is totally normal. There's a float or a poppet inside there that needs the water pressure to seal. So it will leak or flood even til you open the valve up far enough to seal it. No worries.--
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I'm just clarifying here. Do you mean it leaks when the valve is open all the way or it leaks while you're opening it and it stops leaking at approx 3/4 the way?
If I can't fix it, it's broken!