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

When I moved into my house it already had a rainbird pop up sprinkler system in the front yard. It has 2 manual valves in the ground, as well as 2 anti-siphon valves that come out of the ground next to them. One of the valves apparently has no effect, as I only have 1 zone of sprinklers and they all come on when I turn the first valve on. My question is; is it normal that as I turn the sprinklers on the anti-siphon valves poor water out of the top of them until the sprinklers pop up? It normally takes 10-20 seconds as a large amount of water pours out of the top. The anti-siphon valve will thump as it "closes" and the sprinklers all pop up. Thanks for any help you can provide.