2004 Chevy Cavalier Main fuel line has air in it.

Hi, LionHeart here, my mother owns a 2004 Cavalier 2.2L, Check Engine and Trac Off are constanlty on, computer check indicated EVAP System Leak. Lately its been stalling a lot and misfiring which eventually leads the car to total shut down. It will hesitate to start or not start at all after several minutes. To follow protocol I did the following tests: Spark (Good), Air Filter (Replaced), Fuel Filter (Replaced OEM), PCV (Good). After doing all of that the problem persisted. Later that day while inspecting the fuel lines for leaks I spoted a leak where the Fuel Pressure Regulator is at; replaced it, no change. Then my light bulb lit, there could be air in the line, so I tried flushing the line at the plug on top of the engine. To my surprise a burst of bubbly fuel burst came out, and after it a prolonged hissing noise of air. After a few secs it stops, so I crank it to see if the car started, voila, it does. I reaaaally need to know what could be causing this potentialy-expensive-to-fix-problem, for I fear that the either the fuel pump is bad, or some really expensive electronic sensor might be bad. It is the only ride that me and my mother have, please we are very desperate.

I offer my knowledge in Subaru Forester 97-2002 if u need to know something, I have shop manual if interested.