Test your brake fluid before you use is it. Flushed my brakes with brand new brake fluid I had just bought. When the job was finished I used my Scotty approved brake fluid tester in the reservoir and it indicated it needed to be changed. As I was standing there scratching my head I tested the unused portion still in the bottle, yep it was bad also. The lesson is yours.
What is the point of your post?
to test your brake fluid before the flush instead of after. save yourself some extra work
Seals aren't perfect. Brake fluid is hygroscopic, meaning it readily absorbs any water that it encounters. No bottle of brake fluid is filled perfectly to the top, it has some air inside, which contains water vapor. It takes a long time, but eventually that foil covered paper cap that covers the opening of the bottle will be permeated by moisture. The plastic bottle itself will eventually allow tiny amounts of water into the inside as well.
Your car's brake system is like the bottle. It's sealed, but not perfectly sealed. It doesn't allow brake fluid to come out if it's in good shape, but water vapor still slowly gets in there through seals, causing the brake fluid to lose effectiveness and corrode the lines from the inside. This is why you need to flush your brake fluid every once in a while.
An interesting approach to testing brake fluid, though I don't know how accurate it really is:
https://www.youtube.com/watch?v=No5Hmk9Q_6A
thats informative, my first new car was a 79 AMC Concord,
Those pen testers aren't reliable. I don't trust them one bit.
i tested it on a couple of different mediums