How can I fix a leaking sprinkler valve?
The most likely issue going on here is that the sprinkler piping down stream from the valve is at the same elevation or higher elevation than the valve itself. Water left in the line is flowing back through the anti siphon air gap in the valve and resulting in what you see as a "leak".

Normally sprinkler valves are meant to be installed so that they are eight to twelve inches above the highest elevation of the watering circuit to prevent this problem.