Essential to the braking system, brake fluid is usually a mineral oil or silicone based formula with a high boiling point. It is the medium by which the hydraulic pressure from the brake master cylinder is transferred to the brakes. As a general rule, brake fluids should be replaced every two years. This is due to the fact that, over time, brake fluid absorbs moisture from the environment and becomes corrosive to the braking system. Significant levels of moisture will also cause the brake fluid to boil, making braking ineffective and spongy. The fluid level will drop as the brake pads wear and the calipers or wheel cylinders extend further to compensate.