Which method is a quick way to isolate a defective master cylinder with no external leaks?

Pumping up the brake pedal and holding it firm is a reliable method for isolating a defective master cylinder, especially when no external leaks are present. When you pump the brake pedal, you build up hydraulic pressure within the brake system. If the pedal holds firm and does not sink under steady pressure, it indicates that the master cylinder is functioning correctly and is not allowing fluid to bypass internally. Conversely, if the pedal sinks over time while being held, it suggests that there might be an internal leak in the master cylinder itself, allowing brake fluid to bypass and resulting in diminished pressure needed to operate the brakes effectively.

The other methods mentioned do provide useful information but do not specifically isolate the master cylinder. Checking the brake fluid level is important for ensuring there’s enough fluid in the system, but it does not confirm whether the master cylinder is functioning properly. Inspecting brake lines for wear or damage is essential for overall safety but would not reveal issues related to the master cylinder directly. Testing brake light function is also important but primarily relates to the electrical system rather than the hydraulic components involved in braking. Thus, none of these methods directly assess the integrity of the master cylinder like the pedal pumping method does.