The first thing I would check to see is if there is some dirt/debris inside the valve itself. To do this, you will need to turn off the supply line to the irrigation system and open up the valve. There should be a rubber diaphram in it. Take this out and check to see if there is any debris in the body. While you are at it, check the diaphram itself for any pin hole leaks. Reassemble everything and see if your leak is still happening.
