The problem is not the power steering pump. Your steering rack seals have failed and you need to rebuild or replace the steering rack. This is a common problem with the Taurus/Sable of your vintage. To confirm, check out the tie rod boots. I am willing to bet that when the car is running the boots fill with fluid. The rack boots actually fill while
driving, draining the reservoir and if the boot(s) don't rupture, the fluid runs back into the reservoir when the engine is shut down and causes the fluid to squirt out the top.