Changing the PS fluid is something that always makes me want a mityvac.
That said, in my experience, I needed to fill at least ten times to really get fluid through the system. And even then, there was significant residue.
2006 GMC Sierra 2500HD 4WD Duramax LBZ/Allison 6-speed
2002 BMW M3 Alpinweiß/Black
1999 323i KP/GTS2 Alpinweiß
1990 325is Brilliantrot/Tan
1989 325is S50B30US Alpinweiß/Black
1989 M3 Alpinweiß/Black S50B32 (321hp, ITBs)