OK, here goes...
After a good bit of faffing about (you really don't want to know), the new "late Xantia" type suspension regulators are now fitted to the 2.5 XM, and this morning I did a road test.
Have to report that it really is rather good. Previously, the XM's suspension system was in very good shape, with all spheres at correct standard pressures, and working as well as it should, so there is something sensible with which to compare.
Left home along a km or so of uneven and rocky dirt rack, which I must admit felt a bit knobbly, reached our local village, and got on to tarmac. A few minutes later, the suspension noticeably settled down as some trapped air (the front corner spheres had been removed) worked its way out of the system. The drive down 8km of winding tarmac mountain road to the coast was lovely. Along it, I threw the car into a couple of fast bends, and the transition to Firm and back again (dash LEDs for the EVs) was spot on, and almost imperceptible - just as it should be.
Then on to the coastal motorway, and 15km to the next exit, some at 110km/hr cruise, and a bit at 140. Perfect ride, even over undulations and bridge-joints. Flooring the accelerator brought on a nice smooth change to Firm, again aok.
Back along the coast road: local traffic, lights, raised pedestrian crossings, speed humps and ridges. This was the real test, simply because this collection of moderate-speed lumps, bumps and ridges is exactly what Hydractive II has always tackled less than perfectly.
Well, it's better. Extended speed bumps (raised pedestrian crossings, two or three metres in length, and maybe 12cm high) can be taken somewhat faster, and without the system annoyingly going firm while negotiating them. I say "going firm", but this was previously not normally shown by the EV LEDs extinguishing. In other words, the EVs remained energised (Soft), but the ride told you different.
This is exactly what Simon identified in his excellent analysis of the new-type valves - that they should, theoretically, be more immune to system pressure fluctuations and ridge-induced shock-waves, because the regulator shuttle valve is held in position by internal sphere pressure and the new spring, and not by line system pressure. All I can say is, this seems to be so. Unless the car is driven deliberately fast over a raised and elongated hump (when there will be a thump), at decent speeds the car just takes it in its stride, with no crashiness or thump, and, most pleasing, no crash when the back wheels clear the obstacle, just a soft descent. Bingo!
Then back up the 8km of mountain road to our village - all good - and again on to the 1km of rocky dirt track. This last was handled far better than the outward journey, with the suspension nicely mopping up the rocky knobbles. Air-in-system presumably gone, and everything warmed up.
So, it's looking good. Worth the effort of changing over? Yes. It was good before, but now it's subtly better, and that's welcome.
What I will do is put together a technical write-up about fitting the replacement valves. There are a few dodges and cautions, and a good bit was learned along the way.