Gman,

Since you have ruled out a mainline cause of the problem. One of three things could be wrong.

1. I hope this is it becuase this is the cheapest. Make sure your solenoid is securely in place. Not over tightened but snug and not loose.

2. A broken lateral zone pipe that doesn't show up on the surface when the zone water is running. Remove a nozzle from a head and see if the water coming out is discolored. this could also clog other heads. Look at the filters inside the head to see if contamination is occurring. Opening the head to 360 degrees won't show the problem; it only puts more demand on an already overtaxed line.

3. Most likely, the valve is faulty. Unscrew the the top off and see if there is something jamming the blade from opening all the way up. If there is nothing obvious just replace the valve and see if that works. This is more time consuming than expensive. If you want you could put in a larger size valve than what is there to increase your system efficiency while you are at it. Aloha