1992 Other Saturn Models Pedal goes all the way to the floor

I've been having a problem with my brakes ever since the rear passenger's side brake line cracked from being too rusty. Here's a list of parts that have been replaced: Both wheel cylinders, both brake shoes, both drums, both rear brake lines (one was cracked, other was replaced as preventive maintenance), and the front driver's side caliper. The problem is that when the car is running, and the brake pedal is depressed, the pedal goes all the way to the floor. After each repair, I have bled all four brake lines. After the most recent repair, which was replacing the rear driver's side brake line, I bled all four lines again, and still the pedal pushes all the way to the floor. I used a vacuum tool to bleed the lines this time around. I am positive that all the air has been removed from the brake lines.

The only thing I can think of is the brake lines that were replaced had to be flared and connected from separate pieces by couplings. I cannot find any leaking fluid anywhere to suggest air is coming in. Is it possible that I did not properly bleed the line? However, I made sure that as I was applying vacuum to the line, it never went above atmospheric pressure, and plenty of brake fluid passed through each line. Would it be time to replace the master cylinder, or possibly the power booster? Thank you very much for your answer.