I'd start by unhooking the high pressure line going into the steering box. See if you can then turn the pump with the pressure relieved. Perhaps there's a blockage? Is the filter in one piece?

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

The filter in the original and replacement pump are fine, nothing suspect in either case.

I would guess that there's a blockage somewhere, but where I don't know, and what would cause it I cannot say. I have been looking at EPC for hints as to where the problem might come from. The W140 is supposed to have parameter steering which I assume is accomplished through fluid regulation, but what does the regulating I don't know yet.

I might have to pull the high pressure line and see what's going on as you suggest. I thought about trying compressed air either back through the return line or via the pressure side but if the problem is a regulator valve it would likely do no good.

Thanks
Steve A

__________________
Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others