> All of this helps to explain why many rebuilt engines have air dryers hanging on them.

Air drying apparatus goes back almost 100 years, and those air dryers are relatively recent additions. Compressing air heats it, and as it cools, moisture condenses out of it. The cooling apparatus (radiating pipes between the compressor and the first main reservoir) helps collect the moisture in a defined location where it flows into the first main reservoir, which must be drained periodically (manually or by those automatic blow downs and spitters) so its internal volume of doesn't materially decrease. Air for the feed valve is drawn off the second main reservoir and is a lot dryer. Extended response times to brake applications in cold weather is more related to the cold gumming up the lubricants used in the control valves.