you need to clean out the valve because there is something in there that is holding it open.....to start turn off you main water supply.....then locate your eleectric valves and open the one up that controlls the zone that has the problem.....clean out the valve and flush it throughouly to remove any debris and res assemble turn on the water and check it out. Also look to make sure that there are no holes or cracks in the valve interior fittings
Thanks
Ray
Rays Above and Beyond Automatic Lawn Sprinklers