I flushed the reservoir with original Volvo fluid and I've been topping it off with Pentosin.

Is there anything I can add to stop the seals from leaking?

Is a rebuild possible?

NA 2004 v70 240,000 miles

Try a stop leak product, unless it is really leaking badly. Rebuild racks are available.
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My name is Klaus and I am a V♂lv♂holic

Any specific brand, or product? I'm nearly certain adding mineral oil based fluid when they specify pentosin destroys the rack.

I've been to several auto parts stores already. Everyone has fluid with a stop-leak additive to swell the seals, but they are all mineral oil based fluid.

Is there petroleum based steering fluid with a stop leak additive?
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I'm stuck on Volvo and Volvo's stuck on me....

You are correct, I didn't look at the year of your car. The 11S is significantly thinner than the old fluids, so it works better in the winter. There is a lot of discussion on what to do about seal leaks on the web:
http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1062287

and which oils should be used. There is even one where the O ring came apart on the PS pump hose, so find the source of the leak. You can get the Pentosin at O'riellys.
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My name is Klaus and I am a V♂lv♂holic

i used a Lucas steering stop-leak/additive on my 1995 850 a couple years ago, no leaks since then. it is a little heavy/sluggish on cold mornings but its better than a repair AND it aint leaking no more
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-woodsy; 245k 1995 850 wagon; 123k 2000 V70