Abstract: A pump comprising at least a pump body having a chamber (110) communication with an inlet duct (130) and an outlet duct (140) via respective inlet and outlet valves (136, 146), a piston (120) moveable in sealed manner within said chamber, a cyclical actuator (400) of the rotary cam type for co-operating with said piston to cause it to slide, and a motor (500) for driving the cam in an unchanging predetermined direction while injecting volumes greater than the cylinder capacity of the pump, wherein the pump includes control means (404, 220, 600) adapted, on detecting that a required volume has been injected, to determine the position occupied by the rotary cam and to return the piston to the initial position of its delivery stroke prior to injecting a following volume by reversing the direction of rotation of the motor during reinitialization of the piston whenever the rotary cam is positioned on its delivery slope on such detection, and in contrast reinitializing the piston without reversing the direction of