By way of belated update, I thoroughly cleaned out the backflow pressure valve components about a week or so ago, and the eruptions of water just stopped.
So I guess it just needed a "tune up" cleaning!
I wire brushed and dremeled out the crud, which was caked on a half millimeter thick, and that seems to be what the problem was.
I guess one of the three valves were sticking from the corrosion. SMALL:
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LARGE:
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To put it back together, I followed my own DIY from many years ago, archived over here:
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Which brings you to here:
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Where I searched to find this thread from
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$20checkvalve$20backflow|sort:relevance/alt.home.repair/PWa8EZbbyps/PnRfJLPK7SYJ What would happen if I just bypassed this garden water check valve?
11/5/2012 by Danny D.
No sense in taking new pictures - they look almost exactly the same.
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