Well if there's air in the fuel at that point, then you know your problem is upstream from there. If there's no air, then you know the problem is after the Injection pump. This is just a point of reference ,(Like a sight glass) to see what's going on. If you don't see any fuel moving at all, you may have ah clog in the filter block, or a lift pump issue. As (rsh4364) said ah fuel pressure gauge would also be ah big help in pin pointing what the issue is. Have you tried pressurizing the fuel tank with an air hose and ah rag? This can tell you if there's ah leak between the lift pump and the tank. Or if the lift pump isn't working. Just ah coupe ideas, hope this helps, let us know.