A brake system for a vehicle including: a brake caliper; a brake fluid container connected to a brake pipe; a brake hose connecting the brake pipe to the brake caliper; a refractometer arranged in the proximity of the brake caliper and configured to measure a refraction of the brake fluid at the brake caliper; a temperature sensor arranged in the proximity of the refractometer and configured to measure a temperature of the brake fluid in the proximity of the brake caliper; and a control unit configured to determine a water content of the brake fluid based on the measured refraction and the measured temperature.