If you push the pedal all the way down while bleeding, you've probably ruined the seals in the master cylinder.
When you are braking in your car, your brake pedal usually only goes about half way down so the piston in the master cylinder only goes part way of its full travel. If the master cyliner is old or the fluid has not been flushed regularly, the far end of the master cylinder can get pitted from rust or have grit in the fluid down the far end. If when bleeding, you push the pedal all the way down, you can cause damage to the cup seal in the master by running it over the rough surface down that end.
If the master has been freshly rebuilt and the bore given a light hone, then it shouldn't be a problem.