This is the third time that I've had a leaking Presta valve on the tube. There must be a way of repairing the Presta valve rather than having to discard a good tube. Someone did it by applying glue, but I didn't watch closely enough to see where he put the glue. What can I do?
You could try rubber cement however, you will probably still have a slow leak.
You best solution is to prevent the valve from separating from the tube. To do this, I don't put the retaining nut on until after I have fully inflated the tire. Once the tire is inflated, I screw the nut on just tight enough that it won't fall off. You could add some loc-tite but I don't and haven't had a problem. If you over tighten the nut, you are going to cause the valve to separate from the tube.