Brake warning light came on after hitting the brakes hard. Won't go off?

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Had to suddenly hit the brakes at a stop light - - someone stopped a bit short in front of me. Hit the pedal hard, truck stopped just fine, but then, the brake warning light on the instrument panel came on and the pedal now goes way down - - almost to the floor. There is fluid in the brake master cylinder reservoir. Did I damage something by hitting the pedal so hard? The front brakes were completely re-built last year, new everything. The rear brakes are fine. Help?