1999 Mercury Sable power steering fluid disapears

Hello. As you see I am new to this forum. I registered in order to help a friend out who owns Ford Taurus. It is a 1999 model, but anyway, he has been having some trouble with his power steering. At first I thought the problem was linked directly to there being air in the line, but after the proper procedure was performed, it didn't help any. Okay though, here is the problem. The power steering pump will remain full with fluid as long as the engine is not running; however, as soon as the engine is cranked, the power steering fluid is sucked direct into the system and the power steering reservoir in emptied completely, but here is were it gets weird. With the engine running we would add powersteering fluid into the reservoir to fill it up, but immediately after he turned the engine off again, the powersteering fluid would begin to overflow out of the top of the reservoir.

It sound crazy, but I saw this with my own eyes. What could be causing this issue, first there is fluid in the reservoir, cut the engine on and then its sucked in, add fluid to fill the reservoir because of that, and then turn the engine off and watch as fluid begins to overflow out the reservoir.