The brake system requires brake fluid that is free from any air bubbles. Because the brakes are operated through hydraulic pressure, air bubbles in the fluid can compress and reduce the force applied to the brakes.
Air bubbles can enter the brake system during normal repairs (such as a brake caliper replacement). Bleeding or flushing out the brake fluid removes any air bubbles from the brake fluid. The modern method uses an appliance known as a brake bleeder. The brake bleeder introduces fresh brake fluid—under pressure—to the brake fluid reservoir.
The brake bleeder screws (located at each brake caliper) are manually opened in succession, allowing fresh fluid to flow through that branch. The screws are opened in the following order: right rear, left rear, right front, left front, until all four are filled with fresh brake fluid. This process not only fills the reservoir with fresh brake fluid, but also naturally purges and displaces any trapped air, thus "bleeding" the brakes.
Flushing the brake fluid is recommended at time or mileage intervals, based on the manufacturer's recommended maintenance schedule. In most modern vehicles, the brake fluid level is monitored via a sensor in the brake fluid reservoir. Changes in the brake fluid level are displayed on the instrument panel as an indicator of normal brake wear. As the brake pads or brake shoes wear, the brake fluid level will correspondingly drop from maximum to minimum levels. When the brake fluid level nears minimum—and the low-fluid display activates—the brakes should be inspected for wear before adding brake fluid. If the brakes are not worn and the fluid level is low, there may be a leak. Inspect and repair accordingly.