Most glycol-based brake fluids are intentionally hygroscopic: they absorb small amounts of moisture so water doesn’t pool in calipers or lines, where it could freeze or cause localized corrosion. Moisture gradually enters via vented reservoirs and seals. The trade-off is boiling point loss over time, so periodic changes matter, especially with Nordic temperature swings and salted roads. The same principle applies to DOT 5.1.