Where is the leak coming from? If it's coming from the soldered connection on the ZV then there may be more to it then just a bad solder joint (unless you KNOW this is what it is).
It could be that the copper has worn thin and now had a pin hole leak in it etc.
You need to sweat copper out of the zone valve and inspect it.
Drain all water from the system. When you refill you need to bleed the air off through the bleeder valves located on the rads (some will have one some won't).
If you're not that comfortable with these systems etc I would strongly recommend you hire a professional.