Are you on a well and what type of flow (gpm) are you running that you need a 1.5" PVB. Are the valves also 1.5"

Thanks for taking the time to reply. Yes I'm on a well, the incoming and outgoing PVB is 1.5", but tomorrow I'll check the valves and stations to give you an accurate reply on those. (Pretty sure it's stepped down to about an inch). I have a method to determine the GPM (I know it's 20+) but it is involved, I'll do it if you think it's important. The system has supplied 6- 360 degree 30' hunters/zone very well for the last 23 years. This one zone however has far more and much smaller nozzles.

Just for your information, a 1" Pressure Vaccuum Breaker would have been ample size for 20+ GPM.
You stated that all other zones operating through that PVB are operating correctly. That should eliminate any partially closed, clogged shut off valve or filter which may be located before the PVB. A PVB needs a certain amt. of pressure at the poppet to keep it sealed and prevent flushing water from under the bonnet assy. That being said, I,m thinking that you may have a rather large leak somewhere in the zone piping. What may be happening is that when you start that zone the pump pressure switch is at its high end but as the water travels
through the zone and the break/leak? the flow is increased because of the break and as the flow increases the pressure switch is moving to it lower end and maybe below it. This may cause the poppet in the PVB to loose its seal and drop down, causing water to gush from the PVB. Watch the guage when the zone is operating and see if this is what is happening.

Well, good news. I found the leak. I did exactly what was recommended, and blocked all 25 (low volume) emitters in zone one, and did a walk through after turning the zone on. But the mystery continues. Initially the entire zone was bone dry, so I went back to the source, the PVB. And water was bubbling out of the ground about 2 feet from the PVB. The PVB was not pulsing water as before, even after several minutes. The reason I didn't see the leak before, is because when the PVB was pulsating large amounts of water out, it was over the area where the leak was. The 'Mystery' as I see it, is that the leak is long before the zone one valve. And the remaining nine zones still operate normally, with absolutely no water at all leaking out of area I found severely leaking when zone one is on. Could be that only zone one builds up enough pressure to separate a big crack in the PVC underground. Give me a few days to repair the leak and recheck everything, and I'll let everyone know what I found and if everything is back to normal.

Waiting to hear what was done to fix your problem. Let us Know

Sorry about the delay. Christmas and visiting family was (still is) a bit overwhelming. Hope the pictures come through (3). The leak was in fact confined to zone one. (All mysteries are now solved). There were a few more 'emitters' that I had forgotten about. (About 20 total in the one narrow but long winding area). As you can see, a weed (it was attractive, looked like a some kind of palm, so stupidly let it grow) grew so big it cracked the 1" PVC supply line.
Cut the root out with a chain saw and repaired the break. Everything working normal now. I am so grateful for the education and recommendations you all gave. This kind of forum is a real blessing.

Photo 2

Photo # 3. The fix!

a "mystery" no more, good job