Most brake fluid used in production vehicles today is glycol-based. This type of fluid is "hygroscopic", which means it naturally absorbs moisture that's present in the air at all times. According to engineers, that absorption rate is approximately 1.5 to 3% a year in areas of normal atmospheric pressures. In humid climates, that rate can climb even higher. Moisture will always find its way into the lines through microscopic pores in brake hoses, seams, joints, and seals - there's simply no way to avoid it.
As brake fluid absorbs more and more water, it will begin to boil at lower and lower temperatures. Remember that brakes work by converting energy - forward motion - into friction, which is dissipated as heat. The heat in the pads and rotors eventually makes its way to the brake fluid in the hoses and lines.
In normal liquid form, brake fluid does not compress - this allows pressure in the system to be transferred consistently and evenly. Boiling is bad because when fluid changes into a gaseous state and aerates into bubbles, it does compress - creating a squishy-feeling pedal and reduced stopping power. In a worse-case scenario, the brake pedal may sink to the floor, giving you no braking at all. Performing a brake fluid flush prevents this gradual decay in the effectiveness of your braking system.This chart shows how the boiling point of brake fluid is affected by percent of water content. Further below in this article is a chart showing the exact boiling points of the various grades of brake fluid.
While water in the brake system is the main reason fluid flushes are performed, the hygroscopic nature of brake fluid actually proves helpful in preventing other types of problems. For example, if brake fluid was not hygroscopic, water (which is heavier by weight) would pool together in pockets and gravitate to lower areas in the system such as the brake calipers. This would create a more serious corrosion problem, and those pockets of water would boil much sooner than when water is dispersed evenly throughout all of the fluid.
It's also important to note that corrosion of metal brake lines and moving parts such as calipers and master cylinder pistons can and will occur eventually if moisture in old brake fluid builds to a significant point. These metal parts can actually corrode from the inside out. A brake line which fails can lead to partial or complete brake failure.
Gritty corrosion on parts surfaces causes seals to wear and leak, and it causes calipers to bind up to the point where they no longer apply or release the brakes effectively. Preventing corrosion of expensive-to-replace brake system components is another important reason you should perform brake fluid flushes at the intervals recommended by your vehicle manufacturer.