Shaking while braking usually means that you have an issue with your rotors. It's not necessary what is going on with it, but that is the typical answer to the problem. Overtime your brake pads can transfer material to the brake rotor and when this happens it can cause high and low spots on your brake rotor which in turn is where the shaking comes from when braking. It could also be that your rotors are warped - that the could have gotten overly hot at some point and the metal is no longer "true".
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