Combine first 4 ingredients in a 6-quart pressure cooker. Bring to a boil; reduce heat, and simmer 5 minutes. Remove 1/2 cup chili sauce mixture from pressure cooker
Trim fat from brisket. Rub the brisket with pepper and garlic. Place in pressure cooker. Spoon some chili sauce over brisket.
Close lid securely; bring to high pressure over high heat (about 5 minutes). Adjust the heat to medium or level needed to maintain high pressure; cook 1 hour
Remove from heat; place pressure cooker under cold running water until pressure is released completely. Remove lid.
Remove brisket and set aside. Add the brown sugar to chili sauce mixture in pressure cooker; bring to a boil. Reduce heat and simmer, uncovered, for 5 minutes, stirring frequently.
Shred the brisket using 2 forks. Return meat to sauce in pressure cooker; cook until thoroughly heated. Spoon meat onto buns and serve.