Guidelines for brake fluid replacement
Brake fluid should be changed every two years. If the car is driven in a manner that requires frequent and heavy braking, such as driving in mountainous terrain or in a tropical climate with high humidity, the fluid should be changed every year.
Background
The oil must be replaced because it is hygroscopic (the fluid absorbs water molecules from the air). This absorption of water occurs through the brake hoses and brake fluid reservoir. These components are rubber and plastic respectively and are not completely sealed against water molecules. An increased water content in the brake fluid dramatically reduces its boiling point and causes corrosion in the brake system components. This results in damage to the brake system. Braking performance deteriorates markedly.
Use only Volvo Genuine brake fluid (Volvo DOT 4+). Never reuse brake fluid. Always keep brake fluid well sealed in its original packaging.
Continually check that there is sufficient brake fluid in the brake fluid reservoir when replacing brake fluid.