Had a valve today do something strange that I've never encountered before. It's a 2" Irritrol Century Plus and I opened the actuator to check out a sprinkler's adjustment. It fired manually like normal and approximately four minutes later it suddenly just shut down. I went back to the valve, closed and then reopened the actuator and the valve wouldn't fire. Made sure the pitot tube and bottom holes were free of debris or other restrictions. Went to the clock and attempted to fire it but again it wouldn't come on. (I'm certain the valve had been working from the controller because the zone was amply green.) Only way it would fire (either manually or automatically) was by opening the bleeder screw. Ended up changing it out with a top end job and all is well again. Any guesses as to what happened?