Find the brake fluid reservoir / brake fluid tank and check fluid level
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Checking the brake fluid in your 1999 Toyota 4Runner is easy and should be done once a month. On some Toyotas, the owners manual suggests pumping the brake pedal 25 times before looking at the fluid level, be sure to check your owners manual to see if this applies to your 1999 4Runner. In addition to checking the brake fluid level, you want to look at the color of the brake fluid in your 4Runner - if it is dark in color or it has been more than 2 years since you have changed it, you should consider having it changed - this will help make the internal components (which are expensive) last much longer.
If you need to add fluid to your 4Runner, it is important to check your owners manual for the correct type to add - it likely be either DOT 3, DOT 4 or DOT 5, also known as silicone brake fluid. Be sure to discard bottles of brake fluid that are over a year old - brake fluid sucks the moisture out of the air and the water vapor in the fluid can quickly rust out the brake lines in your 4Runner.