Boots is correct. The way the unit is designed to function is to create a gap (air gap) when there is a possibility of a back syphon-age. You may have a broken line or even some damaged heads. Check them all to see that they are working properly. you might even want to reset the back-flow device.
This link should be helpful... ( www.febcoonline.com/pdf/980001.pdf )
Josh
Irrigation /Landscape Lighting / Pump and Well Specialist