Are the drums turned and clean of scoring? Full surface area is the key to preventing brake fade.
What is the contact area of the shoe's arc? Remove the drums and see what portion of the shoe is actually doing the work.
Are the shoes and drums clean? For this you must also remove the drums and inspect. The shoe linings and the drum surface area should be clean enough to eat off of. Any accumulation of grease will compromise the braking. making you have to apply the brakes harder and resulting in more brake fade. Potential sources of grease are the wheel seals and the brake adjusters, as well as grease added to the rub points between the shoe and the backing plate.
Are the drums round? You have to remove the track and axle shafts to test for that. Watch the fingers around those ice scrapers though if you are going to spin the drum around to test.
What are your driving habits? If you are constantly going past 90° then brake fade will be the result eventually. If you are dragging the brakes for long periods as opposed to a short hit, that may also be the problem.
Last, and this is kind of the wildcard: What are the composition of your linings. Since we don't have the luxery of the old school materials, the new stuff is not necessarily better.
Long story short, you will have to pull the drums to know for sure. Even though they may have been done not that long ago, you have the symptoms of a problem.
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