There is never any reason to add brake fluid to the reservoir. Brake fluid may be deliquescent ( takes moisture from the air),buy it doesn't evaporate. The reservoir is large enough to accomodate all 8 brake pads to wear to the minimum. When you push the caliper pistons back, the fluid level rises back to normal in the reservoir. When the red brake light flashes or comes on solid, you have one of two conditions. Your pads are worn down or you have developed a brake fluid leak. If you do develop a serious leak, don't panic, because modern cars have two isolated circuits. Brake efficiency will be reduced, but you will always have brakes at two wheels to get you to the shop safely as long as you realize that you are driving with defective brakes. To recap, there is no reason to top off brake fluid. When the red light starts flashing or comes on there is a reason and you should have the brakes checked.
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