Place the roast in the slow cooker and cover water. Cook on low for five hours. Remove the meat and while still warm, shred it.
At four and a half hours into the slow cooking, add the butter, sugar, ketchup, A-1 sauce, onion, beef stock, Worchestershire sauce, apple cider vinegar, garlic, and lemon juice to a small sauce pan. Bring to a simmer and cook for 30 minutes uncovered.
Drain any left over water from the slow cooker.
Add the shredded beef back to the slow cooker and cover it with the sauce. Place the top on, slightly a jar. Cook on low for 2 hours, stirring 2 to 3 times during cooking. Serve on sandwich buns.