G'day mate does your pump have a vacuum diaphragm on the front of the pump (should have two pipes going up to the top of the inlet manifold where the hose from the aircleaner joins it) ? I think it should from memory, if so then the diaphragm is shot. It is the same diaphragm as a fordson major and are available aftermarket from any one that deals with sparex or bepco.
Easy to fit give it a GOOD clean down where the diaphragm housing mounts to the pump and undo the two pipes, then the four bolts that go through the housing into the main body of the inj pump. When you get it off you will see how to fit the new rubber do not adjust the screw in the end of the housing as this is a damper and can be a pain to get right again.
Refit in the reverse order making sure to get the pin in the hook on the end of the rack ( check rack to make sure it slides back and fwd easy first ) .
Hope I have explained it good enough for you to understand.
Good luck
Jon