Brake fluid is hygroscopic, meaning it is able to draw in moisture like a sponge. It is no wonder, then, that the brake fluid in the master cylinder is often dark, almost black in appearance. But does that mean it has to be changed?
Not always. Your mechanic can use test strips or a Digital Multimeter to test the moisture content. On the DMM, Any reading over 0.30 volts is too much, and the fluid should be flushed. This test is a result of an SAE paper showing a distinct relationship between moisture content and the voltage measured.
Copper content is also a measure of the condition of the brake fluid. A greenish tinge to the fluid color indicates excessive copper content. A test strip can verify copper in the fluid. This, too, is cause for a fluid flush.
Brake fluid that has outlived its useful life can lead to more expensive repairs and impact brake system function. Make a habit of asking your mechanic to check fluid condition as part of your normal vehicle inspection.
Note: This test method is a "quick" test that does not meet any SAE Best Practice standard. High measurements recorded by the DMM should be followed up by testing with a brake fluid refractometer or dedicated fluid tester. The use of test strips is also a good choice.
The resistance change that occurs in brake fluid as moisture content increases is well documented. It is this research that has led to the development of dedicated electronic fluid testers. While moisture content is certainly an issue, it is the measure of copper content that is most recommended as a basis for fluid replacement. Here's more detail on the subject:
"Corrosion inhibitors, pH stabilizers and antioxidants are added to brake fluid to improve the long-term corrosion protection of brake systems.
Over time these corrosion inhibitors can become depleted leaving the internal parts of the brake system vulnerable to corrosion. There are many variables involved in determining how long it takes to deplete the corrosion inhibitors including brake fluid chemistry, chemical and thermal stability, brake system design, driving habits of the operator, frequency of maintenance, temperature, and road surfaces. Another unrelated extensive study found that the buffer capacity and inhibitor concentrations "drop to less than 10% of their initial levels after only 30 months of service". (Jackson, SAE paper 971007,Corrosion Prevention SP-1265, 1997)
"Copper has direct role in the corrosion of the brake system, as well as providing an indirect relationship to the age of the brake fluid.
In the NIST report, Ricker et al , hypothesizes "the copper in the brake lines corrodes at a slow rate over several months or years resulting in copper ions in the brake fluid. These ions then act as oxidizers and plate out in the ABS valves when the corrosion inhibitors can no longer prevent corrosion of the ferrous components. According to this hypothesis, copper corrosion starts when the vehicle is new and proceeds at a rate that is limited by the oxidizer content of the brake fluid, mass transport of this oxidizer, and the effectiveness of the corrosion inhibitors in the brake fluid at retarding copper corrosion."
Copper is the first or "Alpha Contaminate" and will corrode before other metals in brake system according to Ricker et al  because "even though copper is in galvanic contact with more active metals, the low conductivity of the brake fluid allows copper corrosion to proceed."
You might be asking how does copper get in the brake fluid? The answer is from the brake lines. The inside surface of the brake lines is coated with a copper brazing alloy. Brake lines represent perhaps the largest surface area in the brake system. The rate of corrosion of the copper is dependent on the level of corrosion inhibitors in the brake fluid.
This copper corrosion acts as an early warning for the next type of corrosion that will appear in the system. One of the reports cited by the task force, noted the following results:
"Dissolved iron appears in the brake fluid after the initial ammine corrosion inhibitors are significantly depleted and dissolved copper levels rise to around 200 ppm. According to the same report, by the time you see significant levels of iron in the brake fluid, severe corrosion is active. The moisture content of the brake fluid also plays a role in this process, but not the way you might think. According to the report, as brake fluid absorbs moisture, it will result in the precipitation of copper oxides and the formation of hydrogen ions causing corrosion of iron components. Once iron corrosion begins, copper acts as a catalyst to speed corrosion."
Based on the reports reviewed by the task force, the following position can be taken in regards to copper contamination of the brake fluid:
Copper ions in brake fluid have been proven to:
1. interfere with proper ABS valve operation;
2. act as an oxidation catalyst;
3. provide a precursor warning to active iron corrosion;
4. correlate to the age and mileage of vehicular service; and
5. correlate to the buffering capability of the brake fluid.
Based on this, the task force adopted the following in regards to when to recommend brake fluid flushing and has incorporated them in to a Motorist Assurance Program's (MAP) Uniform Inspection and Communication Standards:
SUGGESTION: Test brake fluid at OE-recommended brake system inspection service intervals to insure copper content is below 200 ppm.
REQUIREMENT: Replace brake fluid if copper content exceeds 200 ppm.
SUGGESTION: Replace brake fluid at vehicle-specific OE replacement intervals (if they exist).
Article taken from a blog on www.SearchAutoParts.com