As an avid 'Do It Yourselfer' I love solving a good home maintenance mystery, usually with the help of the Internet. But I have an irrigation issue that absolutely defies any explanation. Just one zone of the ten in my yard's system will turn on normally and run for as long as 1 minute 15 seconds, and then pressure dies while the Pressure Vacuum Breaker pulses large gushes of water from just under the cap onto the ground. This happens whether I open the valve manually or with the controller. Adjusting the flow rate up or down also makes no difference. The remaining nine zones work normally. So far everyone I've talked to maintains that it is not the PVB because the other nine zones work normally, and it is not the zone valve diaphragm, because it works normally initially. It must be a leak they say, but I have searched extensively any kind of leak or wet spots and cannot find any. Plus I've had several leaks before and the PVB did not gush water nor did the zone operate normally for any length of time before malfunctioning. The system is 23 years old, I built it myself, and has the following components: Rainbird PVB 150 (1.5") and Randel 216PR zone valves. The three issues that I feel make this problem different are: #1: The PVB gushes water during the malfunction. #2: Manually opening the valve makes no difference. #3: The system operates normally for up to 1 1/4 minutes before failing. I'd welcome any opinions or ideas that might solve this mystery but so far I can find no one on the Internet that has had an identical experience.