I just did a rear drum to disc conversion and bleed all my brakes until no air bubbles come from the bleeders. When I finished, the brake pedal drops to the floor when I press it. Before the job it only went down maybe an inch at most. Does anyone have a procedure for bleeding the master cylindar? It is the only thing I can figure mught be wrong. I didn't plug the line when I remove the drums, I just let the fluid bleed out. Also, when I bleed the breaks I pressed the brake pedal to the floor each time I pressed, but now I am reading that doing this may cause damage. How likely is it that the master cylindar is no good?

I would re-check the obvious things like the lines you messed with and make sure they arn't leaking, and that the brake fluid reservoir is full. You might want to bleed to lines for the front brakes as well, depending on how much fluid you let drain out of the lines. If none of this or the following helps, it may be your master cylinder.

Bleeding Brakes (with two people)
One person should be near the brakes, the other in the car. Have the person near the brakes open the bleeder valve. The person in the car should then press the pedal to the floor. Then the person near the brakes should tighten the valve. Person in car should then release the pedal completely. Person in car should open valve again, and the process continues until no air is coming out of the valve.

Bleeding Brakes (with speedbleeders)
With Speedbleeders, you fill the bottle with brake fluid. Submerse the line included in the fluid and connect the other line to the Bleeder Valve. Open the valve, get in the car and press the pedal down, release and repeat until all the air is out of the lines. Make sure that the hose in the bottle stays submerged in brake fluid. Otherwise bubbles will get back in the lines when you release the pedal.