How can it be determined that all air has been purged from a master cylinder brake system?

The determination that all air has been purged from a master cylinder brake system is primarily achieved by assessing the firmness or sponginess of the brake pedal. When air enters the brake lines, it compresses under pressure, causing the brake feel to become soft or spongy. This is an indication that not all air has been purged from the system. Conversely, a firm brake pedal indicates a solid connection throughout the brake fluid lines devoid of air pockets. Thus, a firm feel confirms effective purging of air and proper functioning of the brake system.

While checking fluid levels can indicate surface issues, it does not directly correlate to the presence of air trapped in the system. Inspecting brake lines for leaks addresses a different concern, specifically potential loss of pressure or fluid. Measuring the brake pedal travel can provide insights into overall brake performance but isn't a definitive measure of air presence in the system, as various factors contribute to pedal travel. Therefore, the ability to distinguish the condition of the brake pedal is the most reliable method for confirming that air has been successfully purged from the brake system.