As the pads wear on the S/V40 there will be a drop in fluid level as the calipers are extended further. What happens with the 40s is the cap has a rubber boot inside that gets sucked inward in the fluid reservoir and it pushes against the level sensor causing the light to come on. Correction would be to first check the rear pad wear, it happens faster than most people expect, next remove the cap and push the rubber boot back up into the lid. If you are adding a lot of fluid make sure you look for leaks. An ounce or two with pad wear is not a lot.