YOU CAN NOT USE AN ANTI-SIPHON VALUE BEFORE A VALVE MANIFOLD. Anti-siphon valves are ONLY designed to be used at the valves in in manifold. When you use it as a master valve before the manifold, the valve might work, but the "Anti-siphon" component will NOT provide ANY anti-siphon protection.
If you do not want to use anti-siphon valves for the manifold, then you need to use a PVB (Pressure Vaccum Breaker) Back Flow Preventer between the water source and the valves such as this:
http://www.sprinklerwarehouse.com/Wilkins-WK720A-100-PVB-Assemby-p/wk720a-100.htm