Last year, I was planning on replacing my leaking steering rack. I kept
putting it off and it finally turned cold and I put it off until this summer.
Knowing that transmission fluid is the fluid of choice in the 928 steering
system, I put a bottle of transmission stop-leak in the system, figuring it
couldn't make things worse. I can't believe it but after a few weeks the leak
stopped. I keep checking the reservoir and it's always good.
I'm not recommending this to anyone but it worked for me...so far. If it starts leaking again, I'm just going to order a rebuilt from one of the big three or from AutoZone where (I've heard) many are rebuilt for them, et al.
I've done the transmission stop-leak (Valvoline) on the P/S system and it does work. I don't think you can do much damage to the system as it's only really making the seals expand and making them more compliant. Worst case you rebuild the rack.
Three years ago I sucked out the power steering reservoir and filled up with a bottle of Lucus Power Steering Stop Leak juice.
Still drip free.