I'm with Dave on this one. A dark line the entire length of the printed pages is most likely to be caused by a cartridge defect. If you have a genuine Apple cartridge, Apple has not offered those for sale in a long time, so seals and gaskets may be wearing out, even if it has lots of toner left. I have a cartridge with a dark line, and keep it on hand for printing bulky manuals where I don't care too much about print quality, but may need lots of pages.
This site lists all the known printers based on that 'marking engine". The cartridges for those other printers will interchange, such as the HP92274A cartridge for the HP laserjet 4MP.
Some (maybe all) printers based on the Canon LBP-PX engine:
Apple LaserWriter 4/600 PS
Apple Personal LaserWriter 300
Apple Personal LaserWriter 320
Canon LBP-4 U
Canon LBP-430
Canon LBP-430w
Canon LBP-PX
Canon LBP-PX II
HP C2003a LaserJet 4L
HP LaserJet 4L
HP LaserJet 4ML
HP LaserJet 4MP
HP LaserJet 4P
Lasermaster Winprinter 1000
from
http://www.fixyourownprinter.com/reference/pcr/engine/777
Message was edited by: Grant Bennet-Alder