A liquid delivery system includes a first reservoir having a first outlet through which liquid is delivered. A first delivery outlet is located at the lower portion of the first reservoir and a vent is included at its upper portion. A second reservoir located above the first reservoir and has a second outlet located in its lower portion. A first delivery conduit connects the second outlet to the first reservoir and a second delivery conduit connects the upper portion of the first reservoir with the upper portion of the interior of the second reservoir. A vent valve controls communication through the second delivery conduit. A level sensor senses the level of liquid in the first reservoir and causes the vent valve to open when the liquid falls below a first predetermined level and to close the vent valve when on the level rises above a second (higher) predetermined level.