Hmmm.... I serviced my brakes on my 2005 silverado for the first time a few months ago... The truck has 20,000 miles on it. Well I wanted to replace the fluid. I used a mity vac to get the fluid out of the master cylinder and added new dot 4 fluid(large bottle to cover all the wheels), then had the woman pump as I opened each one at a time. I will say this, for a vehicle that is 5 years old with low miles, all the fluid except the master cylinder plastic tank, was all black. So what does that mean? I think it means in my eyes, you must change your brake fluid out because it looked bad, maybe was good, but I don't like the way it looked. The only way I knew it was dirty was to change it. Also I am in AZ and the vehicle was garage kept.... So I don't see how water got in there and I change my own oil and never opened the master cylinder because it was on the full mark on the tank.