Your car shakes some times when slowing down?

If you mean that the brakes "pulsate" when you apply light to moderate pressure... that's caused by a warped rotor. If a rotor overheats it will warp causing disk brake "chatter".

Sometimes the rotors can be turned to make them "true" again, but often you need to replace the rotors. I generally do both front rotors at the same time so that there is no difference between the left and right brakes.