Bubbles/froth in PS resivior [88 90Q]

[Matt Evans]

Thanks to those who responded to my charcoal canister question :)

I put in some fresh pentosin and ran the motor for a while, turne dthe wheel,
even went out for a drive.  Still get a huge amount of frothy bubbling
from the PS resivoir and it overflows and goes everywhere.  

I did some forum searching on audiworld.com and apparently this is caused
by air getting into the system from somewhere.. usually the power steering
pump.  PO of car said he put in a "new" steering rack.  The pump sure
does groan a lot :/  If its the pump thats letting in air, how do i make sure
of this ?  Is there a way to test this apart from just swapping with a 
known good ($$) pump ?  Does the air leak usually occur from insufficient
torque on the banjo bolts (presumably on the "draw" side of the pump?)
.. or does it actually get sucked in from the mating surface between the
front face and the pump body ? (or from somewhere else i don't recognize)

Thanks,
Matt