hi have been having problems with my system i have recently replaced some of my old rads beacause i had a leaking one and some knackered old ones, i have a one pipe feed system and some of the older rads are not getting hot at all which i have narrowed down to prbably a blockedge , however the leaking one i had which has now been replaced with a new double keeps getting air in it and even after a bleed it does not get fully hot, it is only this rad which gets the air in would a blockage cause a rad to get air in?? or could i possibly have a leak that is drawing in air????