I had the same problem on my GT.
I had adjusted the rear brakes, and I could feel that the shoes were just
touching ( one click less they were clear, one click more they were too
tight). After about 50km, same symptoms as yours, slowing down and a smell
My guess is that there was enough heat being produced by the rear shoe just
touching the drum to expand the shoes and hand brake linkage inside the drum
enough to 'put the brakes on' just that little bit more so that the friction
became really noticable.
I backed off one click, and no more problems - ( except of course on the
same trip the overdrive wire shorted out at the base of the gear lever and
cooked part of the loom.)
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