Here's a photo of what can happen to brake fluid after 20 years of little to no use. My blue car was driven less than 3000 miles in 20 years. While that may sound good it was apparently bad for the brake cylinders. What you're seeing is not all rust, most of it is crystalized brake fluid, mixed with rust.

I have been told it is best to bleed the brakes every 2-3 years to rid the system of accumulated water regardless whether it is driven or not. Marty

From what I've seen that's a good idea.

ICKY!!! I have a brake "sucker" device these days. Hook it up on an open bleeder and a vacuum pump sucks it out. Easy to use and kinda a one person job. Need the master cyclinder cap off and keep it full! Less messy and no need to pump the brakes. Also does not disturb your brake warning device that are all unhooked on our cars anyway.

Rob