Brother MP-21C Service Manual page 133

Note: Do not touch the Seal rubbers.
Use rubber globes when you handle them.
(3) Insert a new Seal rubber into the adapter and refit the adapter into position in
the IL cartridge.
Note: To avoid unnecessary air bubble, replacement should be done below
the level of the Initial liquid so that initial liquid comes up to the position
of the Seal rubber.
(4) Close the cover.
B-7