SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

I have an automatic sprinkler system, in my new home, with three zones. I have shut off the valves to the first two zones for the season. The third zone is on as I want to water a newly sodded area. The piping that prevents backflow (I don't know if this is called an ASV or backflow device) in my case looks like an inverted U about over a foot above the sprinkler heads. Lately, this U shaped plumbing pops out from the rest of the plumbing sometimes during operation and water gushes out of the pipes and floods the yard. The meeting point of the U with the rest of the sprinkler plumbing is not threaded but just plugged onto it through some pvc piping. I often pop this back in press it in place and restart the sprinkler. It works this way sometimes but often it pops right out, making me rush back to my IRRITROL control panel to shut of the flow. There is an octagonal brass lid that is screwed on top of the U that looks to me like it is openable, but I don't know what it is supposed to do, and besides, I have tried to open it only to go purple with effort! Please advise ASAP as I don't know what I have or what I should be doing. I don't want to lose my 2500 sqft of newly laid sod!! Thanx.

Pete, Ray, Thanks for your response. I looked through the link "Introduction to cross connection control" and found that the schematic for an AVB (automatic vacuum breaker) resembles what I have the closest. I am not 100% sure this is indeed what it is. I however have a couple of pictures of the device I am talking about that I'd like to share with you so that you can see what I am talking about. Is there any way I can send these to you? Thanx,

NRLP