Check the entire length of the system pipelines under the car - they should not be corroded, kinked or pinched. Wet black dirt at the joints indicates a leak. Brake hoses must be flexible.
Wet black dirt on calipers, bleeder fittings and hose connections also indicates a loss of tightness.
Check if mudguards are installed on all bleeder fittings.
Another way to check: with force (about 300 N) press the brake pedal. Even after a few minutes, the resistance on it should not weaken. Otherwise, the system is not tight or the master cylinder is not working properly.