Press the brake pedal ten times with the engine off to create the same pressure in the cavities of the vacuum booster, close to atmospheric.
While holding the brake pedal down, start the engine. With a working vacuum booster, the brake pedal after starting the engine should «go ahead».
If the pedal is not «goes ahead». check the condition and fastening of the non-return valve and the vacuum hose, as well as the condition of the rubber ring that ensures the tightness of the connection between the vacuum booster and the master cylinder.
If there are no defects, connect a vacuum gauge between the vacuum booster and the check valve and measure the vacuum coming from the intake manifold, which should be at least 40 kg / cm2 when the engine is warm at idle.