Abstract: A rotary positive-displacement pump in which the vane or vanes separating the chambers are locked in the inner dead center point after a certain outlet pressure has been attained, in order to shut off the pump. A corresponding locking apparatus has a detent which assures that each vane is locked in a form-locking manner in the position which shuts off the pumping action. Preferably, the locking apparatus has an overstroke step by means of which it is assured that each vane is constantly held at an increased distance from its sliding surface so that friction and impact noises are positively avoided.