A piston pump assembly for a hydraulic power vehicle braking system including an electric motor, a planetary gear, a worm gear, and a piston displaceable in a cylinder. For an anti-rotation protection of the piston in the cylinder, the piston includes a ring rotationally-fixed thereto by a form fit and connected thereto by forming, including semicircular grooves at its outer circumference, in which cylinder pins engage, which are situated fixed axially-parallel in the cylinder.