Toyota-Lexus-Subaru-Honda: Brake Hydraulic Flush

The standard Dot-4 brake hydraulic fluid that Toyota, Lexus, and Scion and most other manufacturers use is hygroscopic-that is to say, it actively absorbs moisture out of the air, which then becomes a corrosive contaminant.
Toyota is silent on service intervals for the brake hydraulics, which is odd, because Lexus-their sister company-recommends that it be flushed every 30,000 miles, which seems to be a common (though not universal) industry standard elsewhere as well.

The brake fluid is exposed to atmospheric air through the vents in the reservoir cap. Over time, it absorbs water out of the air, which is then dispersed throughout the system, and which is harmful to the insides of the hydraulic brake lines, master cylinder, wheel cylinders, and calipers.

The presence of water in the brake fluid also lowers the boiling point of your brake fluid. When your brake fluid boils, it creates vapor pockets in the lines and/or cylinders. The hydraulic brake system is dependent on having non-compressible fluid to apply the brakes with, not vapor pockets that will compress and so fail to generate the pressures needed to apply the brakes. Your brakes generate an incredible amount of heat. In cases of prolonged braking-say coming down a long curvy mountain road-that heat applied to the moisture in the fluid can result in the fluid boiling and a nearly complete loss of braking ability when you need it most.
It should be pointed out that flushing your brake hydraulics is strictly an act of long-term preventative maintenance. Under normal circumstances with normal braking, your brakes won’t feel any different after flushing them than before.

The brake fluid starts out clear and almost completely without color. It gets darker and transitions to amber and eventually black with age. In the absence of service records, this serves as a casual indicator of age. Just as a casual aside, the clutch hydraulic fluid, which is identical to the brake fluid, turns black much more quickly than the brake fluid does. I speculate that this is perhaps because of two factors: First, the two systems are each exposed to the air through vent caps with the same sized vents in the reservoir caps. The clutch hydraulic system though has a much smaller amount of fluid in it. As a result, the same total amount of moisture absorbed becomes a much higher percentage of the whole, and so has a greater effect. Secondly, the hydraulic fluid in the brake system is routinely exposed to much higher temperatures, and so has a tendency to cook some of the moisture back out of the fluid. This is just my theory, and I haven’t ever seen it discussed anywhere else.

Make getting a Toyota Brake service flush part of your routine preventative maintenance.