What component should be monitored while bleeding brakes?

Monitoring the master cylinder fluid level while bleeding brakes is crucial because it directly impacts the hydraulic system's effectiveness. During the bleeding process, air is removed from the brake lines to ensure that the braking system functions properly and effectively. If the fluid level in the master cylinder falls below a certain point, it can allow air to enter the system, which would defeat the purpose of bleeding and can cause a spongy brake pedal feel or even brake failure.

Keeping an eye on the fluid level ensures that there is an adequate supply of brake fluid to maintain proper hydraulic pressure. This is vital because low fluid levels can lead to insufficient brake performance and can introduce air back into the brake lines.

Ultimately, the other options—such as brake pad thickness, brake line pressure, and wheel rotation—provide important information about brake performance and maintenance but are not directly related to the bleeding process in the same critical way as monitoring the master cylinder fluid level.