Well, I put this pump back on today to see how it would go. It worked! for about 5 minutes on a test drive and then the system started having whines and rumbles when any power assist was required. It was worth a try and at least, for me, shows why/how the pumps fail.
So it looks like the impeller/pressure chamber design is the cause of the failures. It seems to me that microscopic particle build up on the impeller causes the vanes:
1. to jam closed - no sheared shaft but no pressure; or
2. to jam open - rotor jams and shaft shears as it is designed to do at the weak point where the rotor fits.
I guess this reinforces the notion that it's worth flushing the PS system every 25,000 km or so to reduce/prevent particulate build up.
I'll get a new pump tomorrow and hopefully fix the PS on my car for another 150,000 km.
Last edited by Brissyjohn; 01-16-2013 at 05:16 AM.