Did you buy the valve or did it come with the car? I'd try sending it back. Some people have gotten two or three valves before they find one that hasn't leaked. That's where the other ones have been leaking from. Another option is to take the valve off, and remove that fitting. Try to see if it's an O-ring issue or if a part was machined too small.
Now is the time to replace the fluid with DOT5. You need to get all of the old stuff out of the system first.