So my dad traded his decent little Sunfire for a cr@ppy 2005 GMC Envoy XL. The biggest problem it has, aside from the cracked exhaust manifold, flaky door bottoms, non-working VVT and bouncy rear suspension is the power steering only provides assist in one direction. Judging by what I've found by Googling, it's most likely a problem with the spool valve or a bypass valve in the steering rack itself. Is there some type of additive he can put in the steering flid to help free up that stuck valve or is rack replacement the only repair option?