A cartridge unit 10 for an inkjet printer, with an ink storage compartment 46 and an ink feed system 30 for connection to a printhead 22. The ink storage compartment has a displaceable wall section 48 so that its variable storage volume 46 is variable. The displaceable wall 48 is biased 54 toward expanding the variable storage volume 46 so that there is a negative pressure in the ink storage volume 46, such that ink does not inadvertently leak from the ink ejection nozzles.