Not all trimmers have a fuel return line.
Oneof these days you may come across a primer bulb that has a crack in it and then you'll know what i am talking about. They usually crack on the tip of the bulb, therefore, when you put your finger on it to push, you are basically sealing the crack. When you let off, it will suck air in, not gas. Usualy they don't leak either.
There shouldn't be any air in the lines. The fuel filter helps keep the line deep in the tank. If it comes off, you may suck air.
Hank