My 18-year old 560 had, I'm sure, 18-year old brake fluid in it. No doubt, this led to the damage that required my replacing of all FOUR brake calipers (2 frozen, 2 looked close to it, pitted). The fluid looked like dirty, rusty milk.

After doing some extensive reading on the subject, 5+ year old brake fluid is almost as bad as using water. The boiling point goes down very quickly with just a small % increase of water in the fluid. The stuff (DOT3 and 4, anyway) is extremely hygroscopic and will attract moisture in even a "perfectly sealed" system.

I wore gloves when using my Mityvac to pump my system out - was worried about blisters! But the tool did the job, and it was inexpensive.