A piston pump assembly for a hydraulic power vehicle braking system including an electric motor, a planetary gear set, a helical gear, and a piston which is displaceable in a cylinder. To prevent the piston from rotating in the cylinder, cylinder pins are situated in grooves at an inner side of the cylinder and engage with the recesses in a flange of the piston. Due to reshaping, the grooves are closed at one end, so that the cylinder pins are axially secured.