Brake pedal goes to floor with engine running

You can check a booster off the car. Connect it to vacuum (engine running), push a rod in the input side and see if it follows your motion.

If the pedal goes way down, you have compressibility in the system somewhere, booster or not. That could be an air bubble (most common) or bulging rubber hoses. A leak can also make the pedal slowly sink (and suck in air when released), but brake fluid evaporates slowly, so unlikely you wouldn't see a leak, plus the reservoir level would drop. Most commonly, the problem is not bleeding the MC well. Look at videos on youtube. You need ~50 full strokes to get all the air bubbles out. Air in calipers is the next common problem, especially if installed w/o the bleeds at the top.