A/C line valve leaks, any suggestions?

The little a/c line valve on the drivers side leaks quite a bit, do I have any options other then replaceing the whole line? I bought this line from someone one here, and just recently had it put on and it lasted maybe a month until it started leaking. I put the cap on it and it holds the pressure but the A/C is getting warm.
 
It's a tapered thread inside the high or low pressure valve. A 5/16-24 bolt will screw in, but it isn't tapered thread & won't seal. I can't remember what size it is, assuming 5/16NPT or something like that?
Scott
 
It is not a Schrader valve. And the port is replaceable. PM Al (Methodical). He has posted the part numbers for the valves before.

Good luck.