I have here a refill kit and some empty Brother LC51
cartridges, so might as well try to refill them: but I do
not see where to start, e.g. see no likely insertion point
on the shell (and can feel none under paper labels.)
My printer uses the LC41 -- different, I know but still just an "ink
tank" with no printhead. The LC41 is designed with a vent at the top
of the cartirdge, and a valvestem at the bottom of it. I can simply
refill by holding cartridge upright, take a Kleenex and cover the
valvestem and pressing in on it then injection ink through a syringe
without a needle into the rubber grommet opening. Once it is full
visually, I reverse the process. Never had any problems and no
invasive surgery to the cartridge.
The LC51 is similar in design excepting it is a plastic bladder. You
just need to find the ideal angle to refill so that you don't inject
ink straight through to the vent and valvestem. From looking at
various refill pages for LC51 on the net, I believe if you place the
rubber grommet to the bottom of your view and refill in that
orientation it should work to near fill it.