In addition to what others have said, think about how a presta valve functions. There is no spring (like a shrader) to bring the valve back up to seal. It relies on the pressure in the tube. So when the tube is completely deflated, there is no pressure pushing the core back up to sealed position. You may need to try inflating a little by mouth, or maybe have the valve at 12:00 position, so the core falls closed after a pump stroke.