power steering problems?

[06boblee]

My '54 has a rebuilt(Lares) saginaw ps box (from a "77 W200), and a power steering pump that is a 2014 hemi item (have a 5.7 in truck). I am using ATF-4 for the fluid. The pump is new. I am having loss of power steering at speed (35) and when coming to a stop. At idle it seems to work just fine. Any experts out there have any ideas on what my problem could be?

[just me]

It isn't correctly bled. Low fluid level. Either will cause cavitation.

[W_A_Watson_II]

I wonder if the pump valving in the modern (2014) is compatible with the older box's? Not sure about flow rates either.