The commonest cause of a continuous, low volume leak in a sprinkler head is a small debris particle that is holding the diaphragm of the control valve open. The particle is too large to be flushed beyond the valve, yet so small it may be drawn back up the line when water is not flowing. Therefore it is important to flush the line after disassembling the valve, to clear the proximal line of debris. If that effort fails you will need to replace the diaphragm or perhaps replace the solenoid. When the diaphragm is off, examine the seat for scratches and pits caused by debris binding against it.