A method for testing a hydraulic braking system. The method includes activating a brake master cylinder of the braking system mechatronically, and measuring a set of measured decision ratios pertaining to the distance travelled by the piston of the brake master cylinder to the corresponding pressure developed in the brake fluid. The measured decision ratios are then compared with a set of reference decision ratios. In a case where the measured decision ratios lie within a predetermined range, it is detected that the braking system is correctly filled with the brake fluid. However, in a case where the measured decision ratios lie outside the predetermined range, it is determined that the braking system has not been correctly filled with the brake fluid and some air is present in the brake fluid flow circuit of the braking system.