Slice beef into thin strips. Heat oil over medium heat in large skillet; add beef and heat till no longer pink, stirring often. Add water; lower heat, cover and simmer until beef is tender (depends on cut of beef--15 minutes to 1 hour).
Drain pineapple, reserving juice. In small saucepan, combine brown sugar and cornstarch; add vinegar, soy sauce, salt and reserved pineapple juice. Cook over medium heat, stirring constantly, until clear and thick. Pour over hot beef. Let simmer 10 minutes. Add pineapple chunks and peppers about 2 to 3 minutes before serving, so peppers will stay firm.