I've been refilling Canon printer cartridges for a long time... its an amazing deal...
I've been refilling by leaving the cartridge in the printer, removing rubbery plug and filling with a syringe with a curved tube.
What I discovered today, is that when I remove the rubbery cap, the ink level in the cartridge goes DOWN !!! YIKES.
Sure enough, I pulled the cartridge and there was leaked out ink below the cartridge : (
So, I've changed my process to cover the holes (see arrows) at the top with duct tape BEFORE removing cap. Now I can continue to fill while still in the printer very quickly.