I have a Presta valve that leaks when the "core" is screwed out a few turns. I screw it out 3 turns and i can hear air escaping. On all of the other "cores" i've seen they don't let air escape unless you press down on them. Is this normal?

I suspect that you have a damaged gasket on the little poppet valve that's on the inside of your valve stem.

This is not normal - you probably have a faulty valve, or perhaps there is some foreign matter preventing the valve from closing properly.

The presta valve should not leak, whether the screw is tightened or loose. The valve operates on the principle of differential pressure. When the pressure inside the tube (say 50 psi) is greater than the ambient pressure (14.7 psi), the valve is forced to close. When you pump up a presta valve, you must overcome the pressure inside the tube before air will enter the tube.

Odd. This morning I checked it before my ride and it was not leaking when i unscrewed it. Thanks for the replies. I know it is a simple mech. so i don't think there are a lot of possiblities.

I believe you can actually tighten the "stem", sort of like with a shraeder (sp?) valve.

New tube. End of issue. Ride on.

Ditto

Is it a new tube? If so, have your shop swap it out for a good one.