In a large bowl mix together with your hands hamburger meat, milk, bread crumbs, onion and salt and pepper. Make into patties and quick brown over med to med-hi heat and place into baking pan. Sauce: Mix together ketchup, mustard, vinegar, brown sugar and salt to taste. Heat in a saucepan until hot and pour over hamburgers. Bake at 250 degrees for 2 hours.