An ink reservoir for a printer with an inkjet printhead includes a housing for storing a quantity of ink; an air inlet valve having a ball biased against an aperture of the air inlet valve for sealing an interior of the housing from atmosphere; an ink outlet valve having a circular valve seat and a ball for sealing against the valve seat; and a valve actuator having a sliding shaft extending between the ink outlet valve and the air inlet valve. The valve actuator opens the air inlet valve in response to the ink outlet valve opening to establish fluid communication with the inkjet printhead. The ball in the ink outlet valve is adapted to engage one end of the sliding shaft to slide the shaft towards the air inlet valve, whereat the other end of the shaft engages the ball of the air inlet valve against the bias applied thereon.