Does fuel come out of the relief valve on the injection pump? If It is coming out there and the relief valve is not totally open it may indeed be a injection pump issue. You can test the relief valve with low air pressure. Or clamp the line. Actually that is perhaps the first thing you should do. If the relief valve holds ten pounds of pressure I would think the injection pump may be bad as well.

Of course you mention there is no return fuel from the injection pumps return. So until you find out why it can also be assumed the injection pump is probably still good. The imput to output path inside the injection pump for the base fuel supply is not controlled. Other than for the relief valve I believe on the 603 injection pump. It is a simple cavity to make fuel available to the injection pump elements.