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posted: 9/9/2012 5:31 AM

Brake fluid moisture can cause failure

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Q. Over the past few years, I've had an auto repair shop tell me the brake fluid needed to be flushed in the car since the oxidation level was high. Is this something that needs to be done, or is really unnecessary?

I've been tinkering with cars for about 35 years, and I don't ever recall this problem in cars of yore.

A. Great question! You did not share what kind of car you have or how many miles it has on it, so I will speak to this generally.

The brake fluid system of today's automobile is very complex, especially when equipped with traction control and anti-lock brakes. Even if you do not have these features, it is very important that the brake fluid be clean and free of moisture and other contamination.

Brake fluid is hygroscopic, which means it attracts moisture. Over time brake fluid will accumulate a certain amount of moisture; a study by the National Highway Traffic Safety Administration found that 20 percent of the cars they tested had brake fluid with 5-percent moisture content. A 3-percent moisture content in DOT 3 brake fluid reduces the boiling point of the fluid by more than 100 degrees.

When the moisture in the brake fluid boils because of the tremendous amount of heat generated by the brakes, you can actually lose the ability to stop. In addition to that, the moisture can cause corrosion of the wheel cylinders or brake calipers and eventually cause a leak.

Many car manufacturers call for changing of the brake fluid at 24 to 36 months. I recommend you follow your manufacturer's schedule with one exception; I would change the fluid when other brake service is performed. If you do not know your car's maintenance schedule, you would be safe changing the fluid out every two years.

There are several different ways the fluid can be tested for moisture. One is with a test strip similar to what you would use to test pool water. Another is with an instrument called a refractometer, and lastly there are electronic testers that reveal the boiling point.

The other thing to watch out for is dirty fluid. Brake fluid can get very dirty if left unchanged year after year. I have seen some master cylinders that look as if they were filled with mud. Most likely if the fluid is really dirty looking, it is going to have a high moisture content as well, so either way it will need to be changed.

• Douglas Automotive is at 312 S. Hager Ave., Barrington, (847) 381-0454, and 123 Virginia Road, Crystal Lake, (815) 356-0440. For information, visit Send questions to

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