Brake warning light came on after hitting the brakes hard. Won't go off? on 1998 GMC 2500 Pickup

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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?
(2) Answers
Look for a leak in one of the brk. lines going to the rear brakes, good chance it is between gas tank and frame. They tend to rust there and the extra pressure could have burst the line. Maybe! Have someone pump the pedal while you look under the truck, if leaking you will see it.
Engine off, you may hear it. Other possibilities of course.
If your Emergency Brake light is on you might have a problem with the brake lines. It may just be running lights too!