Brother GTX600NB Instruction Manual page 177

(4) Open the cleaning solution tank lid and fill the tank with the cleaning solution up to above graduation mark 6.
<IMPORTANT>

Take care not to exceed graduation mark 6.
Cleaning solution tank
Lid
Press on the operation panel. Close the cleaning solution tank lid and press
(5)
<IMPORTANT>

After filling the cleaning solution, be sure to close the cleaning solution tank lid.
Precautions to keep good printing quality/use printer over the long term
6-9
6
5
4
3
2
1
X1072
again.