Before replacing the WHOLE valve, try a few things. As Wet Boots sez, make sure the port below the solenoid plunger and the passageway is clear. It seems obvious that water on top of the diaphragm is being replaced nearly as fast as the port will allow it to escape disallowing the valve to open fully. This can be caused by a leaking diaphragm or even a flow control stem turned down.. have a look. A final thing to try is replace the whole valve top, i.e., gut a new valve of the same type and replace all but the body itself. If that doesn't work (considering the few minutes it takes to replace just the guts) then, by all means, replace the whole valve.
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