[motto]

I hesitate to respond not being a carrier owner but as Rob suggested, perhaps driving technique has something to do with the problem or perhaps the way the vehicle is used with many turns over a relatively short distance. From what I know of drum brakes any of them will fade if worked too hard so dragging them is a no no and little jabs are preferable. At speed this must be done very carefully as the vehicle will spin out with little provocation.
Driving a U/C is not as easy as one might think. They can be quite treacherous, I know that much.
If the carrier has track displacement, (did they all have it?) is it fully operational and rigged correctly so as you are not relying entirely on the brakes? i.e.;brakes coming on too soon.
How hot are they when fade becomes apparent? They should function ok until they are damned hot.
Just some random thoughts as I am interested in the topic.

David