A diagnostic method may determine the leak-tightness of at least one seal and/or one valve function in a braking system with at least two brake circuits, following completion of a braking action. The diagnostic method may be carried out at a pressure still present after the braking action in one or both braking circuits and/or in hydraulic line or a working chamber of a piston-cylinder system or at a lower pressure, or it may be carried out at a pressure of at most 30 bar.