[Lynn Eades]

Is it sucking air at your change over tap (or anywhere else on the suction side of the pump.)(flexible hose etc) When you say the diaphragm is good, is it O.K. with modern fuel? Wet the valves in your pump and then put a vacuum gauge on it. It should suck about 15" to 18" of mercury. The delivery should be about 2-2 1/2 psi (which is controlled by the spring) If it does that, and holds it, then work back down your lines, as per my first comments.
That should give you something to think about.