Cut potatoes into 1/2-inch cubes. Place potatoes in a large saucepan; add water to cover. Bring to boiling; reduce heat. Cover and simmer for 5 to 7 minutes or just until tender. Drain well; cool.
In a small saucepan, bring about 2 cups water to boiling. Add green beans; return to boiling. Cover and cook for 3 minutes. Drain; rinse with cold water.
In a very large bowl, combine potatoes, green beans, broccoli and/or cauliflower, and carrot. Add salad dressing and pepper; toss to coat. Cover and chill for 4 to 24 hours. If necessary, stir in enough milk to reach desired consistency. Makes 8 servings.