Leaky irrigation sprinkler even when system off

I have a Hunter in-ground irrigation system. In one of the zones, I have noticed a slow trickle drip down my driveway from one of the sprinklers that continues even after the system has been "off" for several days.
My guess is that perhaps the valve controlling that zone has a very slow leak which would be consistent with the fact that this sprinkler is at the lowest grade of the 4 sprinkler heads in the zone.
Does this diagnosis make sense? If so what typically causes such a leak and can it be repaired? Or do I need to go buy a new valve?
Thanks, Jeff
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You most likely at most need a new diaphragm for the valve. Could be just a grain of sand on the diaphragm.
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Diagnosis is correct. Turn off local manual control valve, take screws off top of valve unit, exposing the rubber diaphragm, clean and flush the unit with water using the manual cutoff valve. Replace parts carefully. As the other poster mentioned, it might just be some dirt in the unit. Could also need a new diaphragm.
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Thanks -- your explanation is very helpful. I will try the above.
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