Brake fluid attracts moisture, over time moisture creeps into the brake hydraulic system and can cause serious problems (especially on today's cars with anti-lock brakes). What happens is the moisture gets into the system and not only causes rust and corrosion in the lines, but it can also reduce the boiling point of the brake fluid and this can cause the fluid to boil inside the caliper causing brake fade.
We had a customer come in for the typical brake job (pads & rotors) and upon the test drive the vehicle pulled to the left and we could tell that something was not right. After checking the calipers for sticking in their slides we eventually found contamination in the brake hydraulic system. When the caliper pistons pushed back in, it literally stirred up the dirt and sediment inside the caliper which kept the pistons from moving freely. The pictures below is of the inside of the brake system and the piston, showing the debris that had accumulated inside the lines causing the problem.
Another repair we make on a regular basis is the replacement of steel brake lines, some of which are rusted and broken from the inside. A brake fluid flush keeps new fluid in the system and greatly decreases the lines from rusting from within and removes debris.
Here is a side by side example of new brake fluid and some fluid that was flushed from a car during a routine brake system flush – please note all of the debris that has settled to the bottom of the used brake fluid bottle. It is a small cost to prevent serious unforeseen braking problems when you need to stop, but this sediment can be deadly for your braking system and must be removed regularly. We recommend flushing the brake fluid every 30 - 40,000 miles for not only the longevity of your braking system parts, but for the safety of your family.