steering issue

previously my power steering had been becoming increasingly tight on right turns only, at first it was only happening from time to time now its constant' this is a US model . I had a local alignment shop check the entire front end, they found nothing . Other than the fact they said I needed a new rack. My question is could this be just a clogged valve in the rack , simply because turning left isn't a problem ., during my inspection of the fluid I noticed it was darker then it should be If I do flush it is there any thing I could use or should i just use the normal pentosin to flush and fill
Greg