Tonight after I manually turned on one of my sprinkler zones for few minutes and after it was done I noticed that water was continuously leaking from the underneath the cap of the anti siphon valve. I thought it would stop after a few minutes, but it did not. When I tried to turn on the sprinkler zone again the water would stop, but after closing it again it would leak again. So I just went ahead and shut off the valve for the whole sprinkler system until I was able to troubleshoot.
So in my situation do I need to replace the whole anti siphon valve unit or can I just fix the cap part? In picture see the part circled in red to indicate where the leak is coming from.