Self Leveling Diagnostic Help

[350SL4spd]

Car is my '83 300TD and the rear stays firmly planted on the ground at all times. (Yes, I searched and have learned much).
The PO (actually the PO's mechanic) stated that there was a fluid leak in the self leveling "on top of the diff somewhere". I have had no leak since I got it home (yes, the reservoir has been full). First I bled the system, then I disconnected the valve by the diff to see if the linkage was stuck. No response from the system moving the switch manually. The pump is pumping enough to bleed the system and if you look in the res w/ it running it's pumping enough to be bringing it back to the res at more than a trickle, but not what I would call "high pressure" (it is a return line).
My question is: How can I determine whether it is the pump or the valve? Is there a pressure spec for the pump? I think I may have be able to jerry-rig up a guage from my fuel pressure set. I know the valve is an easy rebuild and I don't even want to price out a pump...
Thanks!
-M-