Check tightness and serviceability of the brake system

Check under the vehicle piping system along the entire length - they must not be corroded, kinked or pinched. The wet black dirt in the joints indicates a leak. Brake hoses must be flexible.

The wet black dirt on the caliper bleeder valve and hose connection points also points to the loss of tightness.

Check if dirt-protecting covers all the bleeder valve.

Another way to check: a force (300 N), press the brake pedal. Even after a few minutes the resistance to it must not weaken. Otherwise, the system is leaking or defective master cylinder.