This is what the inside of your brake system looks like...

[Tech-GeeK1500]

I have to agree with skylark, I don't understand people that drive their cars everyday in punishing conditions, expose them to the elements, and never think about simple things like fluid changes and then wonder why they have part failures. I just replaced the entire brake system on my truck, I did the NBS master cylinder swap with proportioning valve, all new lines, braided hoses, new calipers, pads, rotors, drums, everything but the ABS module (which I still need to cycle and flush out). Just also did the pitman arm, idler arm, and all the bushings and ball joints in the front end with Moog. Lets hope I'm good for a bit, haha. Seeing the dedication here in these pictures is always inspiring. looks like some amazing work!

 

[77Impala]

On mine someone did the caliper brake hose change not too long before I bought the truck. I have changed both rear wheel cylinders, master cylinder and did the RWAL delete on mine. I did not get any gunk out of the fluid but the fluid was dark before I flushed it out.

 

[21Actual]

Just put all new brakes on my truck. New Master, lines, calipers, wheel cylinders, pads/ rotors, drums/shoes. All shiny and clean on my ride.

I would have to say though, that of all the fluids in a vehicle, changing the brake fluid has to be one of the most overlooked items. The stuff does degrade, and has a service interval for a reason. Nothing but DOT4 for me, and change it out every couple years.