A pump assembly provides for direct replacement of volumes of liquid from
a reservoir with equal volumes of air preferably at substantially
atmospheric pressure, the same pressure or with pressure equalization to
be at least equal to atmospheric pressure. A slide arrangement preferably
positively displaces liquid from the reservoir and air into the
reservoir. The pump draws air from the atmosphere into a chamber from
which the air either is available for passage to replace liquid from the
reservoir or is pressurized to assist dispensing liquid, preferably,
admixing with the liquid to provide foaming. Gravity separation of air
and liquid to be dispensed is used to replace liquid with air in the
reservoir and to selectively place air and liquid into communication with
passageways for ejection.