Brother HL-5280DW - Manual (Page 91)

6. Routine Maintenance
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f Put the toner cartridge firmly into the new drum unit until you hear it lock into place. If you put the cartridge
in properly, the blue lock lever will lift automatically.
CAUTION
Make sure that you put in the toner cartridge properly, or it may separate from the drum unit.
g Put the drum unit and toner cartridge assembly back in the printer.
h Close the front cover.