If the header tank is working fine (is filling) then it will add all the water needed. When you refilled the rads, it may be that you were using water to replace the air quicker than the tank could provide it. Either way, I think the next step is to bleed all the rads in the manner I suggested above..

When you say pressure testing bits of the system, how would you do this if you can't seal off the vent and header tank?