So I read through that other thread. What exactly are your referring to when you mention the
"Hold" position? Is that a specific location of the 3PT hitch lever or is that any place on that continuum so long as it's not moving?
The 3-point control lever has, starting from the front, three positions. Most forward is Down, fluid drains from the 3-point hydraulic cylinder, through this valve, and back to the sump. Pulling the lever up part way gets to what I call Hold, fluid from the hydraulic cylinder can't escape (aside from worn parts) so the lift stays up or wherever it was, indefinitely.
Note the way these hydraulic systems work fluid is still arriving in Hold position, passing through another part of the 3-point valve and draining to the sump. (So if a loader valve is upstream from this 3-point valve, it has pressure available for its functions). Last position of the 3-point lever is when it is pulled all the way back to Lift, the drain to the sump is sealed and all incoming fluid goes to lift the 3-point. If you hold this position after the 3-point reached maximum height, gauge pressure will spike and the 3-point's overpressure relief valve will squeal. And - this creates backpressure that the outlet of the loader valve, upstream, is facing. Instead of pushing fluid through that valve's outlet and toward the 3-point at its maximum rated allowable 500 psi backpressure, the loader valve is instead facing a 2000 psi backpressure downstream that is limited only by the 3-point's relief valve.
In theory this will destroy the loader valve, in practice this is simply a Don't Do This situation and the loader valves seem to survive. One Kubota mechanic posted in that thread that they saw a lot of warranty returns with loaders plumbed in series like this so they quit installing new loaders this way with in-series plumbing. On the other hand, that Yanmar Operation Manual diagram shows series plumbing as how they recommend to install a loader. YMMV. Mine's been plumbed in series, conforming to Yanmar's recommendation, for 35 years and I replaced it only because it dribbled a little (and possibly required more rpm) when lifting at maximum weight. I think this was just wear between the valve's pistons and bores, not anything broken.
Incidentally I was told there aren't o-rings or seals inside a loader valve, just precision tight clearances between piston/bore and that's why it is prohibitively expensive to overhaul a loader valve.
Secondly, what about the case where (I think winston) had where his implement drops slowly and then brings itself back up when needed? Someone in the other thread mentioned having ballast on the rear and working the loader. If the rear ballast is drooping and correcting itself won't that eventually happen at the same time the FEL is being operated? Are those the spikes you're seeing? Having done this for a couple years, have you seen any negative side effects?
There's linkage back there that moves the 3-point control to Lift automatically when the arms droop down. This can cause a minor spike but its only chance this could occur simultaneous with lifting something heavy in the loader. And normal ballast shouldn't require full lift pressure to raise, just some moderate amount of hydraulic pressure. Likewise its seldom that the loader will be working at maximum pressure, that should occur only if you hold a loader valve in Lift (or whatever) position after the loader cylinder has reached the end of its travel. Don't Do This.
So while such spikes can occur, I don't think they're important like moving the Lift and loader controls to the end of their range simultaneously. That's the only condition where pressure in each system, 3-point and loader, is limited only by their separate overload relief valves - causing the hydraulic pump to face a 4000 psi load. Don't Do This and you should be fine.
