If you only fill the small air space with CO₂, it will quickly be absorbed into the beer. You need to keep the CO₂ connected to the keg until it is completely carbonated for the pressure/temp it is at.
If you only fill the small air space with CO₂, it will quickly be absorbed into the beer. You need to keep the CO₂ connected to the keg until it is completely carbonated for the pressure/temp it is at.