http://www.cygnusx1.net/Supra/Librar...ection=SR&P=36
If you have progressive power steering (an option on '89 and later models) the pressure control valve or related parts may be bad. It's a valve used to alternate just how much pressure the power steering system contributes.
I am more inclined to believe that you didn't prime the new rack and pump properly and have a fair amount of air trapped in the system.