Why would an air compressor continue to release air after it has pumped up. Normally an air compressor will pump up and when it shuts down, a short release of air is heard, which comes when the pressure is released from the head.
Bizarre question, but that is what a air compressor does. Once it is full it will release pressure ever time it is triggered to release, once the pressure goes below a preset level the compressor will restart.
In the pressure control switch there is a small valve that is activated when maximum pressure is reached and electric power is disconnected from the motor. This valve relieves the air pressure between the storage tank and the compressor. This keeps the motor from having to start under a full load.