How do you check master cylinder

You pull the cover off and look to see if the brake fluid is at the proper level. If your brake pedal is going to the floor, and the fluid is full, then the master cylinder is bad, If the brake fluid is low, you probably have a leak in a line, caliper, or wheel cylinder.

There was plenty of fluid in the master cylinder ,but the fluid has something mixed in it. I did have to replace a rear wheel cylinder. It started raining so didn't get to finish putting on the shoes. I am worried about the fluid contamination. Any ideas?

Brake fluid can look and does get dirty. They do suggest flushing the whole brake system to get new fluid in. Not many people do this tho it is a good idea to do so. The rear cylinder may have cause the brake pedal to go down if it is leaking bad.