Thanks for the help so far.
I will re-check for debris inside the valve keeping the diaphragm from closing but the valve seems to stay fully open and all four sprinkler heads operate at full bore, not just a trickle as I would expect if debris was in there.
I'll also ensure the solenoid plunger isn't stuck open but this is part of the new valve top so I would hope that the new one didn't have the same problem as the one I changed out.
Spring should be OK - thanks.
The valve doesn't seem to close at all. The control voltage goes but the valve stays (wide) open to give full bore flow at the heads. Valves are Nelson type.
Not sure what a dripline is but I don't think I have one. Pump - valves – pipe work - sprinkler heads, simple as that.
This zone stayed on for about an hour (today) after it should have cut off. Can't see what would cause it to do that as it suggests that the "problem" suddenly goes away??? Would a low pressure surge allow the valve to close?
Any more help gratefully received.