Abstract: A negative pressure drainage vessel equipped with an interior resilient flexible diaphragm and means for producing a negative pressure on that side of such diaphragm which is proximate a fluid recovery area. The opposite side of the diaphragm is exposed to atmospheric pressure. A progressive rise of fluid level in the container, as may tend to increase the pressure on the negative-pressure side of the diaphragm, is accompanied by an elastomeric contraction "pull-down" or "pull-up" of the diaphragm so as to maintain essentially the negative pressure required for an essentially constant suction condition for a large progressive rise in fluid level. The negative pressure may be increased by periodic application of vacuum as may be effected by an air-evacuation mechanism. A separate, liquid-filled bag may be employed together with an appropriate valving means.