A conveyance pump having a substantially constant output pressure. The pump includes a chamber having a circular periphery. A vane is mounted within the pump chamber for oscillation through an arc and a valve chamber includes opening to each boundary of this arc, and with input and output conduits. Within the valve chamber is a valve which may move between two positions, allowing communication between alternate pairs of openings and conduits. An hydraulic motor is connected to the vane to cause its oscillation between the limit positions, and a motor rotates the valve between its positions. In operation the vane will rotate, drawing material from the input opening into the pump chamber behind the advancing vane, and pushing, and thus pressurizing, the material out of the pump chamber in front of the advancing vane and out of the output opening. Reversing the vane continues this process. If a product flow stoppage occurs downstream of the pump the output pressure on the vane will exceed the pressure generated by the hydraulic motor, thus causing the vane to automatically stop oscillation. When the downstream stoppage has been removed, the output pressure will begin to lower, and the vane will again automatically resume oscillation.