Most irrigation valves are designed to close within 10 to 20 seconds of the solenoid deactivating. The most common reason why a valve would close slower than normal is that there is a restriction in the valve metering port(s). Hunter valves incorporate the metering ports in the diaphragm assembly. In the plastic center of the diaphragm assembly, there are small holes that meter (restrict) the amount of water that can flow to the top of the valve. If debris clogs these holes, the flow is restricted too much and the valve can either close real slow or stay open indefinitely. To fix this, simply replace the diaphragm assembly.