Why does this backflow pressure valve constantly leak?

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:

To put it back together, I followed my own DIY from many years ago, archived over here:

Which brings you to here:

Where I searched to find this thread from

No sense in taking new pictures - they look almost exactly the same.

I'll let you know when it leaks next.