(under subclass 212) Apparatus in which the pump has a mechanical pumping member (e.g., rotor or piston) driven from a rotary driving element and an inlet and an outlet; means being provided to change the flow of the pumped fluid without altering the direction of rotation of the driving element, i.e., the inlet under one condition of operation becomes the outlet under the other condition of operation and vice versa; the control means acting on the reversing means to cause its operation.