Place the brisket and juices in a large Dutch oven; add the beef stock, beer and enough water to cover if needed. Bring to a boil, uncovered, and skim off any scum that rises to the surface. Remove from heat and add spice packet, bay leaves and pepper corns, cover and transfer to the oven, braise until very tender about 3 hours and 45 minutes.
Remove from oven and transfer the brisket to a cutting board and cover tightly with foil to keep warm. Add the cabbage, onions, carrots and potatoes and bring to a boil. Lower the heat and simmer until the vegetables are tender, about 20 minutes.
Slice the brisket across the grain of the meat into thin slices. Serve with vegetables topped with cooking liquid. Season to taste.