The term hygroscopic relates to the properties of brake fluids, meaning their natural tendency to absorb moisture (water) from the air. As we all know or should know, water is not a desirable solution to be bringing into a brake system. The absorption of water has two main effects: 1) water causes the onset of rust; rust leads to premature deterioration 2) water lowers the boiling point of brake fluid, causing brake systems to overheat.
The most common symptoms of hygroscopic absorption are the dragging of wheel cylinders and calipers. The dragging results from water not being as good a lubricator for the rubber seals and pistons as the brake fluid, thus a reduction in inelasticity of the rubber seals.
Example: A typical DOT 4 brake fluid has a dry boiling point of 450F right out of the can. A normal brake system annually absorbs 3% of its fluid weight in water, resulting in a reduction of the boiling point to 300F. At the end of year 2, it is very likely that the system absorbs 5% water, bringing the boiling temperature well below 300F.
We prefer for brake fluid not to boil because its physical properties change. As the physical properties of brake fluid changes when it passes through the system (i.e. in the calipers), small bubbles of gas are released. The release of the small bubbles of gas results in a spongy pedal. A spongy pedal can be dangerous because the vehicle will not stop in the appropriate distance.