Well, for starters, on my '93 (w/ ABS, of course) I have never come close to getting a firm pedal. I have bled many times, using vacuum, pressure, and gravity; and I have replaced every component but the ABS modulator and some of the lines, but I have never achieved a pedal firmness, or braking performance, close to a non-ABS Volvo 240.

That said, you do have to "bench" bleed the master cylinder. I do it on the car. Your master cylinder should have come with some plastic fittings and clear tubing to loop the outlets back around into the top of the reservoir, such that pressing the pedal pushes the air out, and the fluid drips back into the resevoir. My master cylinders have always come with the fittings and instructions.

My description of the process may not be the best, but do some searching for "bench bleed" and "master cylinder" and you will find your answer.