In the ink jet printing system described in the specification, a hot melt ink jet printhead having two ink reservoirs is coupled through an ink supply line to a remote hot melt ink supply and a temperature controller is arranged to control the temperatures of the ink in the remote ink supply, the supply line, the ink reservoirs in the printhead and passages leading from the reservoirs to the ink jet orifices at selected temperature levels to inhibit high-temperature degradation of the ink while permitting the ink to be jetted at the desired jetting temperature. In addition, a pressure control system controls the pressure of the ink in the printhead at one or more selected levels to permit the printhead to be used at different orientations and to permit purging of air bubbles and contaminants from the orifice passageways and to supply a relatively high vacuum to a deaerator in the printhead to extract dissolved air from the ink. A check valve in the ink supply line permits the printhead to be positioned at different elevations with respect to the remote ink supply without causing any undesired flow of ink between the reservoir and the printhead.