3d printer binder supply

The HP21/HP22 cartriges are super simple, they are just a box with
some sponge in. The filler hole is covered by a sticker, I read that
they will work with the sticker removed too.

The idea is to just add a tube to the top of the cartrige and connect
this to a little tank allowing gravity to feed the ink.

BUT, if I add a tube and seal it to the cartridge and assuming my tank
has a breather tube will binder travel into the cartridge simply due
to the fact that ink is leaving by the nozzles?

Should I prime the supply tube?

Also does anyone know a simple way to measure viscosity, I don't know
what I am going to try feeding into the cartridge but I'd like it to
seem as much like ink as possible, I'm fairly sure water like
substances just drip out of the nozzles.

Graham