Refilling the original Brother LC-51 cartridges are not among the easiest to do.
I have a Brother MFC-240c that uses the LC-51 cartridges and have found refilling the aftermarket cartridges from the Printer Filling Station are quite a bit easier. The cartridges are not in a casing (shell) that is difficult to separate and the ink is not located in a bag with a plastic skeletal structure. The cartridges also hold more than twice as much ink as the OEM Brother cartridges.
The cartridges are located at the bottom of this page:
http://www.printerfillingstation.com/Ink_Refills/Brother/07B.htm
and refill like this.

1) Remove the cartridge when it is running low or out of ink.

2) Drill a small hole in the top of the cartridge.

3) Fill your syringe full of the proper ink. The cartridge will take a good 35ml of ink.

4) Put the syringe needle into the drilled hole and Slowly inject ink until the cartridge is full again. The cartridges are transparent so there is no problem to see when the cartridge is full of ink.

5) After the cartridge is full of ink, seal the drilled hole with a drop of hot glue or a strip of aluminum tape (not scotch tape)

  That's it - 'drill', 'fill', 'seal'.  

6) Replace the cartridge(s) and run 1 - 3 cleaning cycles as per printer instructions.
Or print the following pages:
http://www.refillinstructions.com/black.htm  for Black
http://www.refillinstructions.com/color.htm  for Color 

www.PrinterFillingStation.com
Printer ink and Refill kits for your Printer