Abstract: A cylinder houses a slidable piston that is separated into two axially spaced portions. A rod interconnects the portions and permits lost motion between the portions when the piston is slid in either direction in the cylinder. By valving and conduits, the piston and cylinder draw quanta of air and liquid mix, for ice milk or the like, together in selected proportion, mix these together and forward them from the pump, e.g. to a freezing chamber. Means are provided, for instance the rod may be exchanged for one of another size, for varying said selected proportion. Preferably a sensor is provided to cause the pump to operate intermittently, e.g. to replenish mix which has been withdrawn from the freezing chamber. Also by preference the freezing chamber is maintained at 10-40 p.s.i.g. in order to tend to prevent reduction of overrun between the time when the air is mixed with the liquid mix and the time when the frozen confection is dispensed.