Brake fluid is one of the most important components in the brake system. Brake fluid is formulated to tolerate moisture absorption, control rubber expansion and corrosion, and acts as a lubricant. It also must not boil or freeze in brake systems over a wide range of operating temperatures. Typically the level is checked when you get an oil change but very seldom does it get completely replaced unless the vehicles braking system undergoes a major overhaul. Brake fluid deteriorates with age due to moisture and contamination.
Also keep in mind when you use your brakes, heat is generated at the friction contact points. As your vehicle sits, your brakes cool down. Therefore, over a period of time the heating and cooling action of your brake system will condense moisture in the closed hydraulics system. DOT 3 and DOT 4 brake fluid will absorb that water and keep it from effecting hydraulic components and helps prevent or at least slow down the corrosive effect. Even though brake fluid absorbs moisture, it cannot continue to absorb it indefinitely, which is why it is suggested that you bleed the system and refill with fresh brake fluid once every 2 years or every 24,000 miles, whichever comes first.