When bleeding aircraft brakes, which of the following indicates successful purge of the system?

Successful purging of the braking system is indicated by the firmness of the brake pedals. When air is removed from the brake lines through the bleeding process, the brake fluid becomes more incompressible, resulting in a more solid feel in the brake pedals when applied. Firm brake pedals signify that there is no longer any air trapped in the braking system, which can lead to reduced braking efficiency if not addressed.

The characteristics of brake operation can reveal a lot about the state of the system. For instance, if the pedals remain soft, it suggests that air may still be present in the system, which is undesirable. A fluid color change is not necessarily a reliable indicator of system integrity or effective bleeding, as the visibility of color can be influenced by various factors unrelated to air content. Similarly, while an increase in brake system pressure can indicate operation, it alone doesn’t confirm the successful bleeding of air; it may not accurately reflect the fluid's compressibility. Thus, the firmness of the brake pedals is the definitive factor that confirms the system has been properly purged of air and is functioning as intended.