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?

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

Ok, so I've come to the conclusion that there is probably an obstruction at the steering gear, hopefully where the high pressure side connects to the gearbox but who knows.

I came to this conclusion based on the fact that if you reverse pump direction briefly it relieves the obstruction, which reappears after about a quarter normal revolution of the pump pulley.

I'm going to first pull the high pressure line and check the orifice at the gearbox and then pull the return line and use compressed air to reverse fluid flow out through the steering gear. Finally, I'll detach the high side from the pump and drain / clean the hose out.

I can't imagine what got in there but certainly something did. If this fails to resolve the problem I'll have to move to plan B, which entails swapping the steering box itself, which means another trip to the salvage yard.

Any other advice or recommendations?

Steve A