If its one of the connecting nuts which is weeping slightly, then it is quite likely that the problem lies with the pipe work and / or the olive within the connection which is leaking. A new valve is not guaranteed to stop the leak.
Try and stop the flow of water to the valve. Drain down is the most straight forward but time consuming. However, if there is space and you have access to freezing equipment, you could try shutting both pump valves (unless they are quarter turn valves, in which case they will probably leak if you touch them) and freezing the other two pipes.
Once there is no water flow, undo the leaking joint. If the olive is sunk into the pipe you will need to replace that bit of pipe (and can probably only do that after a drain down) and use a new olive to re-make the joint. If the olive is not sunk into the pipe, wrap five or six turns of PTFE tape around the olive and re-make the joint.
Even if you replace the valve, you should make sure that the olives to each connection are in good order, replacing them and the pipe-work as necessary.
clean it up, wrap PTFE tape around the olive and re-make the joint.