parts for a Rainbird EV-100 can be had under the Irritrol HR1 model. Certain failures of a diaphragm assembly can give you a partially opening valve. Other issues could be in play.
That valve is long out of production, so there might come a time when you just decide to install a new manifold of valves.
Thanks for the comments. After trying another solenoid & cleaning the valve, it's become obvious that the time to replace them has come. As you suggested, it does appear there is a problem with the diaphragm. For what ever it's worth, my system was installed in the summer of 1988 - 23 years ago. Many of the heads & one or two valves have already been replaced.
Best regards, Richard