Place roast beef in a lightly greased slow cooker.
In a medium bowl, combine beef broth, balsamic vinegar, soy sauce, honey, garlic powder, red pepper flakes and black pepper. Pour over roast beef. Cover and cook on low for 4-6 hours, or until beef is tender.
Remove roast from slow cooker and slice or shred. Cover and keep warm.
For the glaze:
Remove 1 1/2 cups of liquid from the slow cooker and pour through a mesh strainer into a medium saucepan.
Stir together cornstarch and water in a small bowl. Whisk into saucepan, along with balsamic vinegar, brown sugar and soy sauce. Bring to a boil, whisking constantly, until mixture thickens. Season with salt and pepper, to taste. If desired, add additional water or cooking liquid for a thinner glaze.