Drip irrigation leaky valves!

We have five valve stations and are having problems with two anti-siphon valves that leak constantly through the cap on the output side. I have cleaned them and replaced them last year with Rain Bird 3/4" anti-siphon valves, yet they still leak. I even increased the height of the risers thinking maybe they were too short. At some point we had a handy man work on something and he increased the water pressure since we have a well and I am wondering if that is what is causing this, though I can't remember if the old valves had a problem even before that since I remember them replacing them before because they had algae and hard water deposits on them from leaking.

So WHY and what can I do? I'm hearing drip drip drip next to our window in the night and it's making me crazy not to mention the excess water being wasted! Help!

PS We bought this house with the system already in play so maybe something was done wrong to begin with?

 

Did the new valves installed last year leak immediately or did they work fine for a while? It's sorta common for there to be minor problems during spring startup after the system has sat idle through the winter. Have you tried replacing the diaphragm?