A refill unit for an ink reservoir that has an inlet valve for receiving refill ink and an outlet valve for fluid communication with an inkjet printhead. The refill unit has a docking assembly for docking with the ink reservoir, an ink bag for storing ink, a valve actuator configured to close the outlet valve of the ink reservoir and subsequently open the inlet valve upon docking the docking assembly with the ink reservoir, an ink delivery outlet assembly for establishing fluid communication between the ink bag and the ink reservoir upon docking the docking assembly with the ink reservoir and, constrictor actuators for increasing pressure in the ink bag upon docking the docking assembly with the ink reservoir, and reducing pressure in the ink reservoir upon disengagement of the docking assembly and the ink reservoir.