A dispenser drain attachment includes a threaded cap having a tapered drain socket and an associated drain tube. The interior of the drain tube has a double diaphragm which seals the drain tube, as well as an elastic gas bellows for dispensing liquid portions by alternating deformation of the diaphragm disks of the double diaphragm, which acts as a one-way valve. Such deformation is the result of pressure conditions which change when the gas bellows is compressed and then again released. The dispenser drain attachment is assembled from only two one-piece structural parts, each of which is produced by a two-component injection molding procedure.