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Brake Fluid Information and Maintenance Requirements
Brake fluid also needs regular attention. Unfortunately for consumers, few know this and as a result, many eventually suffer brake system problems that they might have prevented. Many vehicle manufacturers now include recommendations for periodic brake fluid changes in their manuals, but not many motorists seem to be getting the message.
While in theory brake-fluid changes as preventive maintenance might mean less potential brake repair work at some future date, it actually can mean more preventive work right now for brake-service customers.
Most vehicles on American roads use DOT 3 or DOT 4 brake fluid. (Brake fluids are graded by the Federal Department of Transportation, hence the "DOT.") These brake fluids are chemically similar; the major difference between them is that DOT 4 boils at higher temperatures than DOT 3.
Silicone-based DOT 5 brake fluid is NOT recommended by vehicle makers for use with ABS. And although its resistance to absorbing moisture may sound like a theoretical advantage, in practice it more often means that any water that does get into a hydraulic system may collect in pockets where it may freeze or boil. Also, it does not mix with other brake fluid types.
Standard brake fluid absorbs water over time. On average, in two years it's saturated: it's collected enough moisture to warrant replacement, otherwise hydraulic system parts - including costly ABS components - may suffer internal rust damage. Also, "wet" brake fluid boils more readily than when it's fresh.
Some professionals may sell their customers on a two-year brake fluid replacement cycle, as recommended in several owners' manuals. But while two years may be the average time until saturation, water absorption rates can vary depending on vehicle design, climate and driving conditions. Besides, other foreign materials may also contaminate brake fluid earlier or later, so consumers may want evidence their vehicle is or isn't due for a brake fluid change.
Until recently, refractometers or fluid sample boilers with spectrographic analyzers were needed to determine brake fluid condition. Not many shops or DIY'ers bothered with the investment. Today test strips are available for both moisture content and contamination level.