Why flush the brake fluid?
Many Original Equipment Manufacturers require the brake fluid to be flushed every two years or 30,000 miles (whichever comes first), to keep the warranty in effect. Both the Car Care Council and The Motorist Assurance Program recommend this service as well.
Brake fluid is hygroscopic. This means it absorbs water (humidity) agressively. Normally brake fluid absorbs 1% to 2% of its weight in water per year. The actual amount of water depends on the condition of the system components, most importantly the hoses. It also depends on the type of brake fluid, humidity and driving conditions.
The brake fluid needs to be exchanged for two reasons.
Moisture in the system will cause the fluid to boil at significantly lower tempuratures. If the fluid boils, this could result in brake failure.
Even if the brake system is “sealed”, moisture still enters the system.This is why most brake fluid resevoirs allow the brake fluid level to be checked without removing the cap.
New, DOT 3 (Deparment Of Transportation) brake fluid boils at 401 degrees. With moisture contamination (“wet”) DOT 3 brake fluid boils at 284 degrees.
DOT 4 brake fluid, primarily used in high performance applications, boils at 446 degrees when new, and 311 degrees when “wet”.
Water in the brake fluid causes corrosion of metal components in the braking system.
This corrosion can cause fluid loss, low and soft brake pedal,and premature pad and or shoe wear that may result in damage to the brake rotor or brake drum.
Corrosion caused by moisture in the brakeing system may damage expensive Anti Lock Brake Components.
How does a car owner know when to have fluid changed?
There are 3 basic types of testers.
reagent dip strips
The greatest problem with all three testers is that the fluid is usually tested in the Master Cylinder Resevoir. It is much more difficult to test the other areas of the system. These other areas are much more likely to contain the highest degree of contaminants. It is much more cost effective to flush the fluid regularly to prevent problems.
Note that the sample of new brake fluid, on the right, is clear. The sample on the left is contaminated brake fluid.