Both. Brake fluid is the most overlooked component in the braking system of vehicles and it is one of the most important components in that 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. The level is checked occasionally, but very seldom is brake fluid completely replaced unless the vehicle's braking system undergoes a major overhaul. Most technicians know that brake fluid deteriorates with age due to moisture and contamination. DOT 3 and DOT 4 brake fluid are polyglycol based. This glycol ether blend of fluids is "hygroscopic" which means it attracts and absorbs moisture. This process takes place every time you take the cap off the container or check the fluid in the master cylinder reservoir. Moisture is even absorbed through microscopic pores in rubber seals and hoses in the brake system. 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.