Steering is stiff
 

I have a 91 LS400 and it has been sitting for about 8 months. I changed the powor steering pump since it was leaking from the seal. My problem now, is that it is stiff when making a left or right turn. I did flush as much as the old fluid as possible by using a suction pump in the resorvoir.
Could old fluid be the problem of stiff steering or could I have a different problem? Have anyone incountered this problem?

Thanks!