Hello ss. Your Brunco sounds like it is burning just right. An airtight stove/furnace/boiler means that you have complete control over the air that DOES enter the stove.. This is evident with your description of the fire going to a low burn/idle that is near to going out.. YOU are controling the air to the fire with the combustion [draft] fan and the ashpan door..
If your stove was not airtight, the controls would have less effect on the fire, and you would not be in control of the burn rate or heat output..
Older stoves/boilers/furnaces that were made entirely of cast iron, with little machining in their production, had leaks around doors. hinges,, sections, and sliding air control dampers.. these stoves/furnaces required a watchfull eye to keep from overfiring or burning up the grates..