These types of lines are typically caused by dirt or white out (correction fluid) on the scanner unit, or the corona wire for printing is dirty. This problem is usually resolved by cleaning the scanner glass (also scanner glass strip and white plastic bar if your Brother machine has Automatic document Feeder (ADF)),with a soft lint-free cloth, or cleaning the corona wire.

Please refer to the instructions, "How do I clean the scanner glass?" and "How do I clean the corona wire?"