At this point, since you have replaced the valve, I would now look at a major line break or a pinched line. Do you have any trees in your yard? Are there any trees in the area of concern? I am leaning more toward (also mentioned by BSME) the line pinch. I have seen it many times in my area especially with pine trees and with the older systems.
If this is the case, your best bet (if it is a small area or system) is to redo the zone in question, or you could try and follow the lateral line by digging up areas to test for a pinch. You would just dig up about two feet (to expose the pipe) and cut into it to see if you have water. If you do go further away from the valve about ten feet and try again, if not, go back toward the valve.
Best of luck!
Josh
Irrigation /Landscape Lighting / Pump and Well Specialist