I just replaced my master cylinder on my 1994 ford thunderbird LX with a 4.6L V8. But my brake pedal is still going down to the floor board. What could be the cause of that?

You've bled the lines from back to front, and bled the master cylinder/ It sounds like air in the lines, so you should be able to get streams of fluid with no air at all bleeder screws-start from the furthest away from the master cylinder, to the closest, then do the master cylinder. If that doesn't work, you must have a broken line or a leaking caliper or wheel cylinder.

You will have to use a cross corner bleeding patern

HI,did you bench bleed the master cylinder,before you in stalled it. Then if you did bench bleed master cylinder, did you bleed all brake lines starting at right rear then left rear,then right front,then left front wheel. You have to do all this to get all the air out of lines,and also have you check for any leaks on brake lines,plus wheel cyclinder , bad brake hoses on hoses they also could be roten on the inside not letting fuild flow. If i can help you any more just re post .
Thanks dodge tec