How do I know what brake parts need to be replaced? HELP!

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So, my brakes needed to be changed, and they were grinding for a while. I knew the pads were wearing way down, but then suddenly the brakes stopped working pretty much. When I press on the brake there is no resistance and I can push the petal all the way down easily at which point the car stops, but it is not a fast stop and it feels like it might not stop at all. It basically feels like I have no brakes. How do I know if I will need to replace the calipers, master cylinder, rotors, or all of them? what is the most common thing needing replacement? Will I need to replace all of them?