Why are my sprinklers at the end of the run leaking?

I checked the control valve for that set of sprinklers and it is dry i.e the gasget is apparently not leaking. Should I still put in a new gasget and washer/O ring or will I have to get a new brass antisiphon valve. The current one is not that old. It is very preplexing. Any suggestions will help. I would rather not call in a plumber.