If you have replaced the printer drum on a Brother printer and it's still displaying a "Replace Drum" message, this is likely because the drum counter was not reset
Note: The drum counter needs to be reset whenever the drum is replaced.
The drum counter can be reset by following this procedure:
1. With the printer turned on, open the front cover
2. Press and hold "OK" for about two seconds, Drum Unit should display on the screen.
3. Press OK.
4. Press the up arrow to reset the drum counter
5. Close the front of the printer back up and resume normal operation
The following video outlines this process as well as showing how to replace the drum: