Here's something most people don't know about brake fluid: it's hygroscopic, meaning it actively absorbs moisture from the air through microscopic pores in brake system hoses. Within just two years, brake fluid can absorb up to 3-4% water content, and this has a dramatic effect on performance.
Fresh brake fluid might boil at 230°C, but with 3.7% water contamination, that same fluid's boiling point can plummet to just 155°C. That's why international standards specify both an "Equilibrium Reflux Boiling Point" (ERBP) for dry fluid and a "Wet Equilibrium Reflux Boiling Point" (WERBP) after moisture absorption.
This moisture absorption is also why brake fluid turns darker over time and why it needs replacement even when it "looks fine." The water doesn't just lower boiling points—it also accelerates internal corrosion of brake components and can lead to vapour lock during heavy braking.
Modern formulations like DOT 4 are engineered to exceed both dry and wet boiling point requirements, maintaining safer margins even as moisture inevitably accumulates.
