In the process of doing the work you created an imbalance on the brake system- much as would happen if a line ruptured or a caliper seal leaked enough not to build pressure. The brake failure light is to alert you to this type of problem.

The switch for the light is in the brake junction block down below the master cylinder (follow the lines). The "exploded" view of the internals is displayed below.

I have heard that at times a light on after working on the system sometimes rights itself but I do not know how that might be. You should be able to unscrew the large plugs at the ends of the block and carefully push one way or the other and have the light go out because you will have centered the slide portion of the switch. Good luck.

Randy