Brake Pedal Diagnostics

From my first post “Brake pedal is hard when pressed with engine off but goes to the floor when car is running.”
Yeah, I got that. But "HARD" could mean it just has more resistance. I'm trying to figure out if you may have a hydraulic issue here. How far down can you mash the pedal with the engine OFF? Can you force it to the floor or close? If so, this is likely a hydraulic issue.