in simplest terms, air rises to the top of a liquid. There is a line in the return sysem that goes back to the filter and one to the top of the injection pump housing . aif there are no leaks anywhere the liquid is held in place in the lines by the existing vacuum you get from a sealed system ( think dip a straw into a soda with your finger over the top and pull it out, the liquid comes with it, take the finger off, i.e. create an air leak, and the liquid all drains out )
So long as there are no leak points, the fuel stays in the system, but if there is a leak anywhere, the fuel drains out, and when this happens , then physics kicks in again and creates a siphon effect and also draws fuel out of some vital areas of the pump and filter and injectors causing the primary side of the injectors to loose prime and making it difficult to start