![]() And it freezes and expands in very cold temperatures. It can change state to steam if it gets hot enough, and it does expand significantly with increased tempearture. Water on the other hand is NOT thermally stable. Brake fluid is thermally stable, that is it does not change state to a gas or significantly change its volumn when operating at normal temps. Rather it is the heat index of water - when the brakes get hot they transfer some of that heat to the brake fluid lines and the fluid. ![]() The mechanism that causes deterioriation of the brake fluid properties is not incompressability because constrained water is compressable. After 18 months, the level of contamination can be as high as 3 percent." The higher the ambient humidity is the more likely you have water in your fluid.Īfter a year of service, the brake fluid in the average vehicle MAY contain as much as 2 percent water. The fluid attracts moisture through microscopic pores in rubber hoses, past seals and exposure to the air. The rational is based on teh fact that glycol-based brake fluid starts to absorb moisture from the moment it is put into service. "Many experts have long recommended changing the brake fluid every year or two for preventative maintenance. But this thread motivated me to go do some research. OK, I have never changed brake fluid in any of my cars. My brake fluid, at 4 years and 70,000 miles is starting to test for contaminants, so I am preparing to flush it. Brake fluid can be tested fairly simply for moisture and other contaminants and can, to a lesser degree of accuracy, be visually inspected for cleanliness. This falls into the 3000 mile oil change philosophy of doing it more often than strictly necessary just to be safe. Annual and bi-annual are some intervals that I have heard. Regarding flushing the brake fluid, there is a school of thought that dictates flushing the fluid based on some time period. Hypothetically, for example, if the light stays on continuously for three ignition cycles it would set a code, but if the light comes on intermittently it would not. It is possible, based on the actions of other service codes, to have a light and not a code, as frequently the code requires certain combinations and/or conditions to be set. I have heard that we have a low brake fluid indicator. I am not looking at a service manual, so this is speculative.
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