I have had juddering through a brake pedal was when a disc was warped, or it can happen when braking sharply on damp or wet road or loose surfaces due to ABS letting go and re-applying the brakes to stop tyre skidding. If it is a warped disc it will happen every time the brakes are applied no matter how gently, if it is ABS it will normally only happen when braking hard ( except on mud or loose gravel / snow - when tyres will try to skid ).