Technical Single brake drum overheats

Here's a final update on this overheating issue: after replacing nearly everything, and being worked on by multiple mechanics, it was discovered that the Tee fitting which splits the line to either side was partially occluded on one side resulting in similar pressure but unequal flow. Cleaning out the obstruction fixed the issue. He should have listened to my wife who made a suggestion like this long ago.. Thanks for all the suggestions.