Soft brakes after replacing the front calipers?

I have the car listed above LE. I replaced the front calipers then bled the brakes (rear right, rear left, front right, front left). While bleeding the brakes, the pedal was firm, after I bled all the brakes and turned the car on the pedal goes all the way to the floor. Is there another reason this could be happening other than there being air in the system? (No leaks are coming from the master cylinder) When I bleed the brakes is there a sure way to know that all the air is out?