Combine the brown sugar, chile powder, cumin, celery salt, garlic, and salt and pepper to taste in a small bowl. Rub all over the brisket, then transfer to a 5-to-6-quart slow cooker. Combine 3/4 cup ketchup, the vinegar and Worcestershire sauce in another small bowl. Pour over the brisket along with 3/4 cup water. Cover and cook on low, 8 hours.
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