Can't help with the spark plugs, but all manufacturers recommend replacing brake fluid periodically because it absorbs moisture from the air through the reservoir cap and rubber flex hoses. That moisture promotes corrosion of metal parts and it lowers the boiling point from well over 400 degrees to 212 degrees. That can result in one form of brake when; a soft and lower brake pedal due to that water vaporizing and forming air bubbles that can be compressed. All brake experts know the value in replacing brake fluid, but very few of us actually do it.