It sounds like you have the rod right and the master should be free of air. You have an "under floor vacuum booster" you say, I assume that is a vacuum unit attached directly to the master. If it is separate from the master, as in hydro-vac, it needs to be bled too. You probably know that.
I don't know what the brake arrangement on a 75-81 P30 is but you said the master was for drums in your first post. That would tell me that it had residual valves in the master. If it was for disc front, drum rear, it should have a valve in the port for the original rear drums. That would normally be the port to the front as mounted in the original vehicle (I am not familiar with the P30 arrangement). If you have information that adding more residual valves does not cause difficulties then I am out of suggestions. Someone else will know more and I am pretty sure they will be along to help.
