I have one zone that has just started to act up. There are five Rainbird 42SA spray heads on the zone. All six zones have worked trouble free for the last several years. In the last few days this particular zone has been over pressuring causing the pump to cycle off as the pressure exceeds 60 PSI. As the pressure drops the pump cycles on. This zone has in the past maintained a steady operating pressure of 45 PSI. When the other zones turn on they operate at a steady 40-45 PSI. Where do I begin my troubleshooting at the zone valve (solenoid,valve diaphram)? Help!!!