Do you have a completely level yard? Is the zone that leaks downhill from all the others? If so, you have what we call low head drainage. It is an easy problem to fix with a few added parts. If not, it could be a valve problem, but would be rare. I have only seen a couple of valves that would seep / or leak (not completely closing) and neither were Rain-bird (Hardie Ultra-Flow).
Josh
Irrigation /Landscape Lighting / Pump and Well Specialist