Place the roast into a slow cooker along with the water. Cover, and cook on LOW for 2 to 4 hours, or until beef can be easily shredded with a fork.
Shred the beef, removing fat as you go. Remove 1/2 cup of the broth from the slow cooker, and reserve for later. Add the vinegar, brown sugar, dry mustard, Worcestershire sauce and ketchup. Mix in the salt, pepper, cayenne, and garlic. Stir so that the meat is well coated.
Cover, and continue to cook beef on LOW for an additional 4 to 6 hours. Add the reserved broth only if necessary to maintain moisture. Serve on toasted buns. The meat can be frozen for future use.