This is not an easy thing to do but it was what a Cat mechanic did on my 89 Contessa that had lost prime on a mechanical injection pump due to an air leak. (Died on an on ramp in Denver, had to be towed, bad scene) He installed an inline sight glass coming from the fuel tank prior to the water seperator and observed no air stream in the fuel with the engine running. It required different fittings to adapt the sight glass to different locations. He then installed it on the line going to the engine injection system just after the final filter and hand pump. We could easily see a bubble train in the fuel with the engine running at that location. Shut down and with fitting changes installed it between the pre and final filters and there were no bubbles in the fuel stream there. My leak point was sourced to the fittings, handpump,filter of the final filter/pump assembly. We replaced the final filter and reinstalled the sight glass on the outlet side going to the engine injection pump and it was now bubble free. The cat final filter was a new version and it had been redesigned with a compromise placement of the o-ring in order to replace several different filter PNs with one for cost savings. That compromise design caused an air leak.
Because it is relatively easy I would change the pre and final filters and the Oring set in the water seperator as Gerald mentioned. An O-ring is the most likely source of an air leak and I believe that would eliminate all of those as a possible sources. One thing I noticed is that the Cat mechanic really tightened things well beyond what I would consider safe without breaking fittings or filters but when he was done it was trouble free. He had many years of experience. If you do get to the point of frustration where you have a mechanic temporarily install a sight glass and find no air in the fuel stream then I believe there is a check valve in the return line that is intended to keep fuel from returning after the engine is shut down to prevent hard starting. If nothing else this lets you know how Cat diagnoses the fuel stream for air leak points. Without some diagnostic method to source the leak it is all guesswork.....Hope this helps.