What is an indication that the air has been purged from the aircraft brake system during bleeding?

The indication that the air has been purged from the aircraft brake system during bleeding is characterized by a firm brake pedal feel. When the brake system is bled correctly, all air bubbles that may cause sponginess or softness in the operation of the brake system are removed. A firm pedal indicates that the hydraulic fluid is being effectively transmitted from the master cylinder to the brake calipers without the interference of air, which can compress and lead to a less responsive braking action.

In a properly functioning brake system, a firm pedal means that the hydraulic pressure is efficiently being applied to engage the brakes, allowing for precise control. Conversely, if there is air remaining in the system, the brake pedal may feel soft or spongy, and the spending of excessive pedal travel may occur, indicating insufficient hydraulic pressure for effective braking. Thus, the firm brake feel confirms a successful purge of air from the system.