This device is called "air filter", which is to clean the air coming from air compressor.
All what you need to do is to tighten the clip(lever) at the bottom, so that amount of air leaking will be reduced.
It is normal to keep air leaking all the time a little bit, but not too much.
As a daily cleaning, operator needs to pull this clip(lever) to take out all dust from air cmpressor.