You can install individual checkvales under the spray heads, known as the hunter HCV. Although you need one for each head, they are good to have. They can be used when using Anti-Siphon valves i believe they have to be 6" above the highest sprinkelr head pop-ed down...because that is when backflow would happen anyway. Of course if you have risers (always above ground, they have to be 6" higher then the highest riser. I would purchase the check valves forom sprinkler warehouse.....best price.......also make sure y ou get the 1/2" threads for the sprays
Thanks
Ray
Rays Above and Beyond Automatic Lawn Sprinklers