If the system has been properly powerflushed (bare in mind this is not a two hour job) and the rest of system is sound then air ingress shouldn’t be a problem once system is filled and bled. Sludge or black iron oxide is caused as you know by air getting into the system over time, my guess is your system boiler was originally an open vented and not properly powerflushed when updated, which should be done before a new boiler and again after completion. Installing a filter on the system should be considered as well, this magnet attracts the sludge as it’s circulating and is easily cleaned during a routine service of system.