Abstract: A pump for use with a container of liquid material for dispensing and or atomizing the liquid material in an accurate dose. The apparatus has a movable valve member which is normally biased in a closed position. Upon actuation of the pump, a fluid pressure is caused to act upon the valve member. The ratio of the area of the valve member against which the pressure acts before and after opening of the valve member is selected to be at least 1:1.5, to cause a corresponding reduction of the force necessary to open the valve and dispense the fluid upon actuation, resulting in a full stroke and single puff each time.