and replace the whole top half of the valve (solenoid, spring, diaphragm, etc) with the new one. Takes 5-10 minutes. I don't really know enough to diagnose the cause of the leak. I've tried just replacing a solenoid or a diaphragm and more often that not I just end up having to go back to the store and doing it anyway. Worth the extra 10 bucks to just get the new valve.