I understand 1990 ls400 has a weakness in the power steering pump, they leak. In my case I only had about 62k miles on my ls400 1990 when I decided I did not want to put up with the leaking any more. I purchased a new aftermarket pump on Ebay with the reservoir for about $160. I also bought new hoses note: the pressure hose is in two pieces. I bought them from rockauto.com cheaply. I had my mechanic install everything cost me $200 more. Worked out fine now 1 year later, no leaks, steering smooth, positive, and quiet no problems. Worth it to avoid keep adding fluid and listening to the noise system makes when there is air in it dripping oil on alternator can create problems. To answer your question specifically, the pump was leaking at the pulley bearing shaft seal. Note: there is also a valve installed on the pump that boosts steering when vacuum is low these go bad too. But you don,t have to buy another (a dealer item expensive,) my pump arrived with a screw where this valve would be just make sure you you plug the vacuum lines you can use a screw.