An ink cartridge for an inkjet printer is configured to keep the ink pressure in the printhead constant. The cartridge has a sealed ink storage volume, an ink outlet for sealed fluid communication between the printhead and the ink storage volume, an air bag in the ink storage volume for expanding as ink is drawn through the ink outlet to keep a constant head of ink above the outlet valve. The air inlet for the air bag is a frangible that is ruptured as the cartridge is installed and the ink outlet opens.