Why should I flush my brake fluid every year, you ask?

[240 Ed]

:EDIT: Mercedes Benz Factory Service Recommendation is every two years. :EDIT:

I shall give my standard lecture on brake fluid, for those who may have mercifully missed it! Ha, Ha too late! YOU MUST READ ON...

Brake fluid is hygroscopic = Means it LOVES water.
Huh? where does it get water...?

There is a tiny hole in the master cylinder cap which allows atmospheric pressure to equalize when the level drops due to pad wear. Eventually over the year, the fluid becomes diluted with water. This water can cause rust in the system, but a worse fate than that awaits!
(wait for it)

Brake fluid has a certain boiling point. DOT 3 has a lower point than the later DOT 4. (NEVER USE DOT 3) DOT 5.1 has the highest.
DOT 5 is silicone fluid and does not absorb water, and is not included in this discussion.

Okay, here is what happens. Water dilutes the fluid, right? Right.
Unfortunately, water boils at a lower temp than brake fluid, so the boiling point of the brake fluid is now lowered.
Why is this important?
In normal use, you won't notice the difference. Brake feel stays the same. Even if you were to use straight water!
The problem comes with intense use, say coming down a mountain side with sustained braking (and pulling an enormous 80 ton yacht, say).

Your calipers heat up with the attendant friction (normal). But now, because of the compromised brake fluid, the fluid starts to boil! What happens when the water in the fluid boils?
That's correct, you get STEAM!!! Steam is a gas! gas does NOT work well at all as a hydraulic medium!!!
What happens to your brakes?
NOTHING!
It is known in the trade as "brake fade". (one of several types)
No matter how hard you press on the pedal, you won't get any more brake. You will probably start to speed up as you are going down hill. The press folks call this "...he lost his brakes"

There you have it.
Change your fluid at least once every other year (in dry climates) and if you live in Louisiana or some humid place, once a year.
Pain in the butt, huh.
Your choice, but you are now informed and can now bore other people with said lecture.