A hopper having a bottom opening into an open topped chamber portion of a housing which defines a discharge passageway leading from the chamber to a discharge end portion. A piston is mounted for reciprocatory movements in the chamber portion and passageway. The piston having a pumping surface normal to the direction of its movement and facing the discharge end portion of the passageway. The piston has an opposite surface which slopes angularly upwardly toward the pumping surface. A check valve is disposed in the passageway at the discharge end portion of the passageway.