Rick's Free Auto Repair Advice

Brake fluid flush

Brake fluid flush — is it really necessary?


Your brake fluid’s anti-corrosive properties diminish over time and the fluid itself absorbs water. Water in brake fluid reduces its boiling point and decreases your braking ability, especially in hard braking situations that create the most heat. That’s why you should get a brake fluid flush.

The combined effects of water and no anti-corrosive properties can also result in the internal degradation of brake lines, calipers, wheel cylinders, and ABS valves. In other words, what you save by not performing a brake fluid flush is nothing compared to the cost of replacing corroded brake lines and other major components.

How does water get into brake fluid?

Brake fluid is hygroscopic (water loving), but the hydraulic system is a sealed system. So how does water get in? Three ways:

1) If the brake fluid is topped off with fluid that has been open to the air, that fluid will carry water into the system.
2) If the cap from the master cylinder reservoir is left off, the brake fluid in the reservoir will adsorb water.
3) As the caliper pistons and wheel cylinder pistons move out and in, the pores in the pistons can carry ambient moisture into contact with the brake fluid. Over time, brake fluid can accumulate up to a 4-6% water concentration.

What is brake fluid?

Brake fluid labeled as Department of Transportation (DOT) DOT 3 and DOT 4 is made mostly of glycol-ether with boil lowering additives as well as anti-corrosive additives. Glycol-ether is used because it has a high boiling point and is compatible with the rubber components used in an automotive braking system.

DOT 3 fluid is approximately 80% glycol ether base while DOT 4 typically is 50 to 65% glycol ether base with 20-30 % Borate Ester to resist boiling.  The boiling point of DOT 3 brake fluid is rated at 401°F with no water in it and as low as 284°F if it’s holding water. DOT 4, on the other hand, has a dry boiling point of 450°F and a wet boiling point of 311°F. but absorbs water faster than DOT 3 fluid.

DOT 5 fluid is silicone-based and has the highest boiling point at around 500° and doesn’t absorb water. That’s why it’s used in racing applications.

Are brake fluids interchangeable?

DOT 4 fluid can be used in a vehicle where DOT 3 is specified, but DOT 3 should never be used on a vehicle where DOT 4 is specified because the DOT 3 boiling point will be lower than the brake system’s minimum specifications.

DOT 5 fluid can never be used in vehicles rated for either DOT 3 or DOT 4

When to perform a brake fluid change?

Some carmakers list a mileage/time recommendation to change brake fluid like every 24,000-miles or 2-years, whichever comes first. But other carmakers don’t list any recommendation for performing a brake fluid change.

How to test brake fluid to see if it requires a change

Brake fluid test strips

When dipped into the brake fluid, the strip changes

Two-pack of Phoenxi brand brake fluid test strips to test the condition of brake fluid in a car or truck

Buy a package of brake fluid test strips and test your brake fluid so you don’t get scammed.

color, indicating the level of copper in the fluid. Copper is leached out of the brazing material used to fabricate steel brake lines. As the anti-corrosive additives in brake fluid wear out and the fluid accumulates water, the brew attacks the brake line seams, degrading the interior of the line.

Moisture meter

Moisture meters are simply conductive meters that register the amount of water in the brake fluid.

Rather than perform a brake fluid change based on mileage

Brake fluid moisture tester to determine condition of brake fluid in car or truck

Battery-powered brake fluid moisture tester

or time, you may want to perform your own testing to get a more accurate idea of the condition of your brake fluid.

©, 2021 Rick Muscoplat

Posted on by Rick Muscoplat


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