Still not making sense. I would rule out the packings. Nothing would stop the flow back to tank. The unloading valve releases pressure back to tank so no hydralic functions work this includes lowering by means of the lift lock valve. So iwould suspect the spool hanging up in the valve.

It's manual levers. Jumping off seat, the truck still lowered slowly. But stops a few seconds later after the deadman switch opens. That would mean it's bypassing from spool valve, but stops when load holding valve closes. (By jumping off seat)
If the load continued to drift with dead man switch open, then I might suspect packing leak. Make sense?
I just can't catch it fail. Hard to fix a working truck!

Guess I'm missing something here. Jumping off the seat has nothing to do with packing. Would this unit have finger tip controls? Nothing can get back to tank with out going through the lift spool except the safety down valve. If it's not finger tip then you may have a mis-staging problem. If it is fiber tip then look at the lift pot for problems.

Since I never got it to fail, can't test it. Had return hose unhooked and ready to measure fluid loss but it never faled. Driver said it failed twice in 400 loads!
I'm guessing lower valve intermittently fails and load drops until he jumps off seat, then load holding valve stops it.
In your experience, what usually fails, lower valve or packing in cylinder?

What I don't under stand is the "intermittent" lowering. Questions: What stage does it lower in free lift or secondary? How fast does it lower when reported? Have you done the drift test and return to tank test? Little more information please?