The neck of the bottle might not be sealing properly around the part that goes into it. Air is pushed out between the bottle neck and that little part when you squeeze. And then when you release, it sucks the lost air back in through the squonk pin.
This happens quite often when I do not screw...
Cerakote it, plenty places everywhere that do it these days. Unfortunately the best in the business only does firearms.
Have a look here: http://www.cerakote.co.za/