An automatic, pneumatic pump having a chamber that is gravity fed through a top opening and through a vertical tube with waste liquid. A buoyant plunger sits on an air inlet nipple, which nipple is located at the bottom of the chamber. Liquid entering the chamber causes the plunger to float and thereby to be unseated from the air inlet nipple. The plunger rises in the tube until it closes the top opening. Pressurized air enters the chamber and forces the waste liquid out through a discharge line. When all air and liquid have been discharged, the plunger falls to the bottom of the tube and reseats on the air inlet nipple so that the cycle can start again.