An ink reservoir bag of flexible, plastic sheet material for an ink delivery system for an ink cartridge is constructed of two sheets of plastic material having peripheral edges bonded together to provide an ink chamber therebetween and has separate fill and exit ports opening into the chamber and respectively defined by a fill tube and an exit tube bonded to the bag between the peripheral edges thereof. Flexible tubing is connected to the exit port through a check valve interposed between the port and tubing, and a lure lock is interconnected with the fill tube to provide an injection sight for filling the bag. The bag is filled by connecting the tubing to a vacuum source, evacuating air from the bag and tubing, connecting the fill port to a source of ink and using the negative pressure to draw ink into the bag and tubing.