1979 rabbit diesel air in fuel line

    So i have been driving a 1979 rabbit diesel my grandfather left me as a commuter car. Recently it started misfiring, losing power even if i put the pedal to thefloor, and finally it would die... I found out it was because there is air in the fuel line. The fuel filter is newer 5000-6000 miles, I checked all the fuel lines and cannot see any visible sign of a leak or anything. All of the connections seem to be fine there not loose or cracked. I honestly do not know much of anything about diesel engines. Any ideas would be appreciated im just trying things one by one to figure this out.