So long as the pump is not sucking air & has enough pressure to lift and lower the 3PL , then I would expect that the lift cylinder would be mostly free of air.
Assuming that, then even with a worn pump ( or engine stopped for that matter ), once raised, the lift still should hold considerable load.
If the 3PL collapses under those conditions, you'd be thinking that the lift seals or lift valve are leaking / open.
UTB piston seals can age badly I was told.
Re lift Valve : As I recall, 450 UTB series has a flaw in the draft mode select lever ( small round lever on the outside rear of the main lever quatrant ). This lever selects whether main lever works in draft or position mode. The flaw is that this lever must only be moved while the lift arms are raised. (There is a warning plaque on mudguard).
If u don't the mechanism gets out of wack permananently : ours faulted to an uncalibrated draft mode ie some lift like yours.
My first and only experience with this was many years ago, but involved removing the quadrant , springs ( one bent) , levers and reassembling in the right position. I didn't have a manual so was trial & error. Trivial job if you know how it should be. Should be Fiat manuals out there.
(BTW, That UTB came with our farm and goes well/ no trouble so we've kept and use regularly)
As I recall all the position/ top link draft control is all done in this external cam & link stuff & there is a control shaft going into the internal raise lower valve. You can get to and test operate that shaft without too much bother so as to bypass the draft+position mechanism.
*You would want to keep in a safe operator position to avoid crushing injury in case the hoist magically springs to life*
HTH's ( BTW, take it easy when tightening bolts on a UTB ! )