MISCO Refractometer Q&A Forum

How do you know when to change DOT brake fluid?

It is generally believed that a moisture content of less than 2% is safe for most driving conditions in most parts of the U.S. Conventional wisdom says that a moisture content of 2 to 3% is marginal and that it should absolutely be changed at 3%. You should consult your vehicles owner’s manual or vehicle manufacturer for their recommendations with respect to your specific car. A car generally absorbs about 1% moisture per year of service life. Many motor vehicle companies recommend changing brake fluid every 12-months, others recommend changing every 24-months. The amount of moisture that brake fluid absorbs depends on a number of factors including the type of fluid, humidity, type of brake hoses, condition of the seals and mileage.