Brake fluid reservoir to be very low even if it is filled often

As front pads wear the piston moves out of the caliper bore, this draws down the fluid, and eventually turns the RED das light on for BRAKES. No fluid needs to be added during normal use, the system is designed to turn the light on when the pads are low. When the caliper pistons are pushed back into the bore, the fluid refills the reservoir. If fluid is added instead of the pads being replaced, when the pads are replaced, the fluid will overflow the top of the master cylinder reservoir. If fluid is continually added and disapears, there may be a leak, is the pedal soft or mushy? Low?

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