A 19-year-old man suffered severe injuries late Sunday night after the brakes on his bicycle apparently failed, sending him careening down Mission Hill into the side of a moving car on busy Huntington Avenue, Boston police said. Deputy Superintendent Dennis White declined to immediately identify the victim, but said the man was brought to nearby Brigham and Women’s Hospital. Investigators were summoned to the scene, near the intersection of Huntington and Frawley Street, because of the severity of the man’s injuries. The car, a maroon Ford Mustang with Pennsylvania license plates, was badly damaged, with a large dent and broken window in its driver’s-side door. The car’s side-view mirror had been ripped off and lay in a pile of broken glass on the MBTA Green Line tracks. Nearby, the bicycle was leaned against a police cruiser. Police stopped motorists who had parked their cars near the accident from leaving, saying the area was a crime scene. Huntington Avenue was closed to traffic in both directions while investigators worked.