1975 kz400 petcock vent?

Ive attached some pictures to show the set up that's on the bike now but im not sure if the previous owner did it right. The petcock does leak when its off but thats not a major problem and Im going to leave it till the winter to replace. However, the bike seems to have a hard time pulling gas sometimes. Im wondering if instead of having the two lines out of the petcock go straight to the two fuel inlets on the carbs if one is supposed to be the fuel line and split off to both carbs and the other is a vent line. However, both ends off the petcock let out a decent amount of gas so it makes me question this. Also the petcock only has ON, RES, and OFF. How would I prime it? Usually there's enough in the bowls but there was one time where there wasn't and it was a real pain. Thank you. 

That is a mechanical valve. It is hooked up correctly. When it is set to ON, fuel will flow, so there is no need for a Prime setting. The valve should be turned off whenever you shut the engine off.

Is there any type of vacuum then? Because sometimes it has a hard time pulling gas and im not sure why. 

Petcock looks like the type that works on gravity. Carbs probably have vents. Perhaps one of those black tubes? Gas tank should vent through the gas cap. These usually have a passage or channel through which air can vent through the key hole. If air cant enter gas cant exit. Open the cap next time you feel theres a problem.  Perhaps there is crud blocking inner workings. Bottom section with the nut cast into it should be removable so you can check for crud, water etc.

 

Remove the bottom filter cap and clean it!!!