It's much easier than that - just pull the low pressure return line, pluge the hole on the reservoir (from the outside, and keep dumping in new fluid until it runs clean. If you run out, you can hook the return line back up or sometimes shoot it in the top.

Sounds to me like what happens when you have a worn belt, which I would suspect if it's sat for a while. Might not hurt to replace them all. I really like the dayco top cogs. They last forever and the notches are in the back of the belt, not the front. I've had the originals from 100K miles ago sitting in my trunk and they weren't noticeably cracked the last time I saw them.

Good Luck and let us know what it is.
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1990 740 Turbo, on its way to stock specs, maybe beyond