Regular Maintenance: Brake Fluid

What it Does

Brake fluid is used to apply the brakes to slow and stop your vehicle. This is an example of a hydraulic system that can apply tremendous amounts of force due to liquids being incompressible. When you press on the brake pedal, the master cylinder applies force to the brake fluid, which applies brake force to the brake assemblies at the wheels.

Why This Matters

A properly functioning brake system is essential to your safety and the safety of the drivers, pedestrians, and property around you. Deteriorated brake fluid can affect the safety and reliability of your brake system in multiple ways.

Possible Problems

Water Moisture Absorption:

Brake fluid (both DOT3 and DOT4) will absorb water moisture from humidity in the air. It gets a chance to do this every time the brake fluid reservoir cap is removed. More importantly, brake fluid reservoirs are commonly vented to the atmosphere, which gives the outside air access to the fluid, even in a non-leaking brake system.

Water Moisture Loading:

Brake fluid is hygroscopic, which means that it absorbs water moisture. The boiling point of water is much lower than the boiling point of fresh brake fluid. As the brake fluid absorbs water moisture, it’s boiling point drops toward that of water. Even just a low percentage of water absorption is enough to significantly lower the brake fluid’s boiling point. If this boiling point drops low enough, the fluid can boil under the heat generated by the brakes. This will reduce or eliminate brake application force until the resulting gas cools and condenses back into a liquid. The reason that brake application force is reduced is because gases, unlike liquids, are compressible.

Causes of Excessive Heat:

Possible causes of high or excessive brake system heat are heavy or repeated brake application, repeated or extended braking while going down a slope, and dragging or binding brake components that cause a brake assembly to drag or seize.

Aged and Deteriorated Fluid:

Aged and deteriorated brake fluid will commonly exhibit excessive water moisture content and degraded corrosion inhibitors. These lacking corrosion inhibitors increase the odds of internal system corrosion, which in turn poses the risk of internal or external system leakage due to damaged sealing surfaces.

Lack of Awareness:

DOT 3 is a very commonly used brake fluid, and Honda specifies that it be replaced every 3 years. Toyota does not seem to include this concern in their maintenance schedules, but it does appear to get brought up at Toyota dealerships. We’ve seen it show up in a GM schedule but on a mileage and not a time basis! The idea here being that DOT3 fluid is just that. It’s DOT3 fluid no matter what vehicle it is used in. This is part of the reason that we take Honda’s recommendation seriously and apply it toward other vehicles that also use DOT3 brake fluid. (For those of who you use DOT4, similar concerns apply.)

Recommended Parts Brands