Fill to the maximum line. Secure the cap tightly ... Take it to an auto mechanic to replace your brake fluid if the strip's color falls in the “required service” category.

Use brake fluid test strips or a pen tester to measure brake fluid quality, condition and color. Old or contaminated brake ...

so bubbles in your brake lines act like a very soft spring in that solid column of brake fluid between your foot and the brake calipers or drums. Bleeding the brakes will flush that air out.