Thanks. I looked up "vacuum gun" and the ones I found on the internet were for sucking liquid out of tight places. Can you give me a name and model of a vacuum gun meant to test car vacuum operated devices?
If you go with the filters first, clear as livininharrow suggested, one in front of and one behind the fuel pump. If it's pumping rust, you don't want it ruining the pump as well.
You can always plug the hose and use the vacuum gun to make sure it will hold. You will need seals from DMT. If you're replacing the heater core or the "h" valve pressure check the before installing. I pull the seats and carpet when doing this job.