A method of priming one or more inkjet printheads, the method comprising the steps of: (i) providing a printhead assembly comprising: an ink distribution manifold having an ink inlet and an ink outlet; one or more inkjet printheads mounted on the manifold, each inkjet printhead comprising an array of nozzles; an upstream ink line connected to the ink inlet; and a downstream ink line connected to the ink outlet; (ii) providing an ink chamber in fluid communication with the ink inlet via the upstream ink line; (iii) providing an air pump having a pump outlet in fluid communication with a headspace of the ink chamber, and a pump inlet in fluid communication with the downstream ink line; (iii) actuating the air pump so as to draw ink from the ink chamber, through the manifold and into the downstream ink line, thereby priming the inkjet printheads; (iv) receiving the ink in an expansion chamber in the downstream ink line; and (v) deactuating the pump.