Did you ever get an answer on stopping the leak? I have the identical problem. I bought a new valve, it leaked, I returned it for another and the new one also leaks.

I never did get an answer but did have some luck with doubling up the orange gaskets.

Also had some rubber ones the same size that worked well. Cannot remember where I got them years ago.

Last resort was going to be that silicon automobile gasket material that comes in tubes. Local hardware store said to try that.

Talking about leak on anti-siphon valves, I had it too. Just installed 6 of them in my yards (front/back), every time a station just stop watering, the backflow cause water leaks at the valve for a little while.
I thought that's what they are designed to do to prevent water to flow back to the main water pipe and could get into your house pipes.

Mine do that too but the leak I am talking about is a stream of water shooting out several feet all the time the valve is open. Finally got it to stop.

Dmullen - how did you get it to stop? That's exactly the same problem I have

Thanks,
Mike