What makes it necessary to bleed brakes?

Bleeding brakes is a critical maintenance procedure primarily conducted to ensure that the brake system functions effectively. This process is essential when air bubbles enter the brake line, which can happen during maintenance activities such as replacing brake components. When brake cylinders are replaced, air can become trapped in the hydraulic lines, preventing the brake fluid from providing adequate pressure needed for effective braking. Therefore, bleeding the brakes removes any air that may have entered the system and ensures that the brake fluid can fully transmit force from the brake pedal to the brake components.

While other factors listed, such as worn brake pads, insufficient brake fluid, and overheating brake discs, can impact brake performance, they do not specifically necessitate the bleeding process. Worn brake pads affect braking efficiency but do not involve air in the system. Insufficient brake fluid can lead to a drop in braking performance, but it is not a direct cause for bleeding; rather, it should be topped up. Overheated brake discs can cause brake fade but do not require bleeding unless there is also a related issue with air in the brake lines. Thus, the need to bleed brakes is most directly related to the replacement of brake cylinders due to the potential introduction of air into the hydraulic system.