May not be the emergency brake buzzer. There is another for the rear door that may even be the same buzzer triggered off of another switch. Double-check the rear door and switch. Also, if your bus is not air-brake, it is the Lucas-Girling nightmare, and I can tell you from experience it is fraught with problems.
"Cheese Wagon" <firstname.lastname@example.org>
Former owner - 1989 Ford B700 64-pass Blue Bird (Rest In Peace, Cheese Wagon)