My daughter has a 99Jimmy, her break pedal goes all the way to the floor. We have changed 3 break lines, replaced the MC and the Vacuum break power booster, and still nothing. When the truck is shut off, the pedal is full. When you turn the key, it goes right to the floor. We’ve been working on this for over a week

 

You'd likely do best to try a GMC forum instead of a Ford forum.

 

Did you bench bleed the MC before installation? It's very very difficult to get all of the air out if you don't.