In addition to what the others said, and this actually comes back to what Chuck posted, by draining the system completely, there is a very good chance you may still have air trapped in the master cylinder. You will have to go through the bleeding process a few more times than you would if you were simply replacing a line or wheel cylinder that does not completely empty the system.
To make this as easy as possible, I would suggest using a pressure bleeder. I have found that it's really the best way to completely bleed the system, especially from a dry situation.
