These Anti Siphn valves are crap.  I recall buying these a few years ago at Home Depot and the guy next to me was like, don't buy orbit valves, they are horrible. It was about all Home Depot had that day and he was going elsewhere to look for an alternative brand.  I wish I had too.  I got home that day with 3 of these and read some reviews, but installed them anyway.  I really didn't know a lot about irrigation systems at the time.