Faint printing from Mac but not PC

Please does anyone know why my Brother laser printer HL 5140 prints very faint when printing from a PDF file on a particular disc CD, but prints fine from a PC. Most other printing is ok but a little paler than from a PC. The driver is CUPS; there is no PostScript driver. I have uninstalled the driver & reinstalled from the pre-installed software on Leopard as instructed by Brother, to no avail.