Jim, they followed your suggestion to change the master cylinder first. It didn't fix it. It couldn't.

If you read the posts, even though they were pulling brake fluid through the front bleeders the wheel cylinders were dry. Why? Because the front of the master cylinder was not pushing fluid into the lines. Without fluid, there is no hydraulic pressure. With no hydraulic pressure, the pedal goes to the floor. And if there are no leaks, why is there no pressure? Because the fluid is not under pressure going into the brake lines. And the only place that can happen is in the distribution block.

They fixed the problem, and that's what is important.