This is actually kind of fun every year...like raking leaves! Air in the system can be bled by opening a bleed valve on the upper most radiator or if you have airlocks you can bleed each one.Not knowing your particular radiator, look for a small pipe coming up from the heat pipe in the radiator which has either a screwdriver type slot or something that looks like a tire air nozzle.I think you can do this in a hot or cold system but I always do mine hot just after the circulator stops.Have a sauce pan ready as you open the valve slowly.You will hear air or catch water.If you have the time,I would bleed each radiator or you might be chasing air around for a while.