operating conditions while still maintaining the desired pressure differential across the filter another method of maintaining a pressure differential across the filter is by the use of one or more check valves in the place of the differential pressure regulator 20 the check valves need to have a cracking pressure below the bubble point of the filter material and be able to pass the maximum amount of required air or gas without creating an unacceptable amount of pressure drop an alternative embodiment rather than using a regulator valve to reduce the vacuum level on the air side of the filter provides means to increase the vacuum on the liquid side of the filter this might be done by means of an additional pump 26 to draw air out of the ink tank with appropriate regulation means 20 as in fig4 this could also be achieved by means of a liquid pump 27 between the liquid side of the filter and the ink tank again with appropriate regulation means 20 as in fig5 these options while requiring the additional costs associated with the required pumps eliminate the need for a regulator valve in the air line in systems where the foam contains a volatile liquid the liquid should generally not be maintained under vacuum for such systems where foamy liquids must be dealt with it is preferable to use a liquid pump to pump the foamy mixture into the foam reducing chamber fig6 and 7 illustrate two embodiments which are appropriate for such systems the embodiment in fig6 restricts the flow of air out of the foam reducing.