Why does the air pressure gauge sometimes show a sudden drop when the service brakes are applied for the first time after a long period of non-use?

1. Air is escaping through the safety valve on the supply reservoir
2. The brake chambers are filling with air that was previously empty, causing a temporary volume expansion pressure drop
3. The governor has failed and is venting air to the atmosphere
4. The one-way check valves have failed and are leaking back to the compressor