Place roast in Dutch oven; add water. Bring to a boil. Reduce heat to low; cover and simmer 2 to 3 hours or until tender.
Drain beef, reserving 3 cups liquid. Shred beef; return to Dutch oven. Add brown sugar, mustard, pepper, vinegar and reserved 3 cups liquid. Bring to a boil. Reduce heat to low; cover and simmer 20 minutes.
Add ketchup and Worcestershire sauce to beef mixture. Simmer, uncovered, for 20 to 25 minutes or until of desired consistency. Serve 1/3 cup beef mixture in each bun.