So your Brother Laser Printer is giving you the "Low Toner" warning. Many people find this annoying and so they decide to replace the toner at this stage. So you remove the old toner and insert the new one only to find out after printing a few pages that the "Low Toner" warning re-appears!
This is a common problem and is the same with most Brother Inkjet Printers also. What you need to do is to use the toner completely before you replace it. In other words dont replace it when you get the "Toner Low" message but print untill the printer prompts you to replace the toner or tells you its empty.
The Brother Printers can also be manually reset or else you can reset them by removing the toner and unplugging the printer for about 20 mins. Next turn it back on and replace the toner.
This method also works most of the time. Here are the reset procedures for the Brother HL2130, HL2240 or HL2250 Printers.