The air scoops may be corroded and not letting air escape. You may have to find and replace. There's probably one inside the boiler, and there should be one somewhere on the line outside. There may be a bleed valve internal to the boiler somewhere also. Once you've used all of the bleed valves, and the system has tanken as much water as it can, you will probably need to then set things so the circulator will run, then do again. The instructions may be on their website if you can't find it in the manual. Often, these things come with two manuals...an installation one and an operator one...you may need both if they aren't combined.