Brakes are a sealed system. There shouldn't be air in the system unless something is leaking. If the pedal drops to the floor, either you have air in the system from something leaking or not previously put back together correctly or a part failure at one or more wheels, or the master cylinder may be faulty, any could cause the same symptom. However, air in the system IMO will always cause the pedal to drop and the only way to bring it up is to pump the pedal (because of the air). I would first look at the fluid level. If the master cylinder is full, probably no leak. Check each wheel for fluid/leaks, check the lines too. If you can't find a failure anywhere or any leaks and the fluid level is good.....then try bleeding at each wheel....have a buddy help you. If that fixes it, I'd still be worried on how air got into the system to begin with.