Why is "bleeding the brakes" performed?

Bleeding the brakes is an essential maintenance procedure aimed at removing air bubbles from the brake lines. When air becomes trapped in the brake system, it can lead to a spongy brake pedal feel and reduced braking effectiveness. This occurs because air is compressible, unlike brake fluid. When the brake pedal is pressed, the presence of air in the lines can prevent the fluid from transmitting the force effectively, resulting in a less responsive brake system.

By bleeding the brakes, technicians ensure that only brake fluid, which is incompressible, remains in the lines. This process restores a firm pedal feel and full braking power, essential for safe vehicle operation. The procedure typically involves opening the bleed screws at the brake calipers or wheel cylinders, allowing the air to escape while ensuring that brake fluid does not run low to avoid introducing new air into the system. This is why removing air bubbles directly correlates to achieving a firm pedal and optimal brake performance.