Brother HL-4040CDN - Manual (Page 137)

Routine Maintenance
128
6
Cleaning the corona wires
6
If you have print quality problems, clean the corona wires as follows:
a
Turn off the printer power switch. Press the front cover release button and then open the front cover.
b
Hold the green handle of the drum unit. Lift the drum unit up and then pull it out until it stops.