Hi, all. I’ve been away from this thread for a bit, as I was awaiting a replacement OPV from my retailer, since the original proposed fix did not work for me.
The new part (full OPV) arrived this past Saturday and I installed it. Actually, I only used the spring and plunger from the new part and kept the housing and cap of the original one, as I couldn’t see how those could possibly be faulty. I am happy to report that after several days of testing now, everything appears to be working perfectly. Now, once I engage the pump (either with coffee or a blind basket), the pressure neatly and uniformly rises to 9 bar and stops there; no more overshooting wildly, exhibiting hysteresis, etc.
The part is so simple, it is hard to imagine why changing out the plunger and spring (I don’t think the spring was the culprit) would make such a difference; this seems to agree with @DavecUK’s original thinking that the OPV shouldn’t really do this. I did have one theory (which may of course be way off) for what was happening in my case. On the old plunger, the indentation ring on the green “rubber” bit where it rests against the raised lip at the bottom of the housing is notably off-center and very deep, whereas the new plunger has a much shallower and more centered indentation. I wonder if the old plunger was just seating somewhat incorrectly, such that when first engaged, it would not raise nicely within the housing and would instead be slanted and maybe even wedged or something, necessitating greater pressure to level it out and get full flow through the OPV, accounting for the pressure spikes. Anyway, it’s a theory.
I did smear a very thin layer of Molykote on the plunger as a preventative as well, to hopefully head off any possibility of this happening again.