Hello I will help you with your question,
You can recover the refrigerant and unscrew the valve housing from the line but I have never found one that did not rip the line as I try to remove it so I end up replacing the whole line. Either way the system has to be discharged and evacuated before refilling.
The plastic cap is actually the primary seal (valve is secondary) so if you only have a very small leak that is normal just be sure the rubber seal in the top of the cap is present and screw it on tight.
Let me know what I can do to help you with your question