I have a 98 Nissan Altima and my brakes are completely shot. The pedal goes down to the floor. The brake fluid is full and their appears to be no leaks. Their is no fluid around the master cylinder and I bled both the front and back brake lines. I thought maybe the brake pads were put on incorrectly the first time but I changed them again today, it made a little difference but the pedal is still almost down to te floor. What could this be??

Sounds like you still have air in the system. You may need to bleed it some more. It will start to get tight once the system is bled properly. Did you install the same type of pads?