You check the brake system by pushing hard on the brake pedal for 20 seconds while the car is stationary. The pedal should stop about halfway down and offer firm resistance.
You check the brake servo (vacuum servo) by pushing the brake pedal repeatedly and then starting the engine with the brake pedal pushed down.
If the brake servo works as it should, the brake pedal should move downward slightly as you start the engine. If it does not, something is wrong with the servo, which means you have to push much harder on the brake pedal to achieve maximum braking power.