Q: How do I know what brake parts need to be replaced? HELP! on 2000 Toyota Corolla

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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?
(2) Answers
Without seeing the car, its hard to say exactly what you will need, but it sounds like you wore the pads and rotors down to a point that the pistons were allowed to leave the caliper resulting in a leak. you will need the pads and rotors for that axle (front or rear) and whatever caliper is leaking.