Whisk beef broth, hoisin sauce, 1/2 cup water, vinegar, five-spice powder and garlic powder in a small bowl. Place celery and carrot strips in the bottom of a 6-quart slow cooker. Add the brisket (slicing in half to fit, if necessary); season on all sides with salt and pepper. Pour in beef broth mixture.
Cover. Cook on high heat for 5 hours or on low heat for 9 to 10 hours.
Remove meat from cooking liquid to cutting board. Cool slightly (meat should be very tender).
Skim any fat off top of sauce in cooker and discard. If necessary, increase heat to high and add bok choy to slow cooker. Cook 15 minutes.
Meanwhile, shred beef with two forks, cutting long pieces in half, if desired. Add back to slow cooker. Stir to coat with sauce and serve with soft sandwich buns or warmed flour tortillas.