Brake pedal goes to floor

Most likely. It is important to never push the brake pedal to the floor. Crud and corrosion build up in the lower halves of the bores where the pistons do not normally travel. Running the pedal over half way, whether surprised by a sudden leak, or when pedal-bleeding with a helper, runs the rubber lip seals over that crud and can rip them. That results in a slowly-sinking brake pedal that often does not show up until two or three days later.

I only use gravity-bleeding with no helper. It takes a little longer, but I rarely run into a damaged master cylinder. When you do have to pump the pedal, such as when running the pistons out of the calipers, just be careful to not push the brake pedal over half way to the floor.