My Brother HL-5250dn prints grey lines down the page, sometimes up to 4 evenly spaced.

SOURCE: Dark line down printed page

problem in your fuser assy call to hardware service provider

SOURCE: toner light after 1K pages

On my unit, the toner light was lit steady amber and the Status light was steady red. I tore a small square out of a post it note, off the top (so that it had adhesive on it), and used that to cover the sensor path way found on either side of the toner cartridge. It is a clear plastic hole, centered toward the bottom of either side of the cartridge.

SOURCE: brother hl 5140 drum light comes on then off every 4

With the front cover open, press and hold down Go button until all 4 LEDs are lit. Once all 4 LEDs are lit, release the go button, then close the front cover.

This will reset the drum counter.

SOURCE: I've installed a new drum AND a new toner

Try this....
1: Turn off power and remove toner/drum
2: Tape the little spring loaded lever just inside the printer on the upper left hand side in the *DOWN* position! Do not replace drum yet and...
3: Turn power on to printer and let it cycle. Status/Toner lights will come on as usual.
4: Turn off printer, remove tape from lever, replace drum and turn back on.

SOURCE: Brother HL-5250 printer toner Light on but Cartridge has Toner it

Try this....
1: Turn off power and remove toner/drum
2: Tape the little spring loaded lever just inside the printer on the upper left hand side in the *DOWN* position! Do not replace drum yet and...
3: Turn power on to printer and let it cycle. Status/Toner lights will come on as usual.
4: Turn off printer, remove tape from lever, replace drum and turn back on.