You'll want to troubleshoot the situation here. First things first, you'll want to dig up the troubled valve to expose the problem. Is it for sure leaking from a valve location, or could it be a break in the pipe, on the zone side of the valve, that only leaks when the water to that particular station is on? If it is the valve, check it over, it could be cracked and the solenoid or the bleeder screw could be loose. Check out these things and get back to us. Bottom line is though, your not going to know what you are dealing with until you expose the problem area.
Tony Posey
Ridge Run Landscapes