I am not sure that I fully understand your question.

Airslides that I have experience of are "closed boxes" and hence the venting system needs to be designed to take 100% of the air flow to the device.

If your Airslide is different to this, could explain please?

There may be gas leakage in either direction through the inlet and outlet but that will be dependent on the configuration of the feed stystem, whether it is sealed (eg with a rotary valve) and any pressure differentials between the head space of the airslide and the feed and discharge vessels / equipment. ■