Power brakes problem?

[Crabass]

Not all brake pads/shoes are created equal. The material that the linings are made of varies with manufacturer and product line, and that seems to translate into significant differences in performance. My experience has been that better brakes are more expensive. I use Wagner Thermo-Quiet and have been very pleased-----not that much more expensive and have always been an improvement not only in stopping power but in smooth, predictable modulation as well (disc/drum systems). Have switched six of my seven registered vehicles. Cured the fading problem in my '65 Chevelle non-power drum brake car, although the pedal effort seems to be about the same. Just sharing my personal experience.
That your brakes work until they heat up suggests to me a fade condition. How quickly they fade is largely a function of the lining material composition. Unless you know for sure that you have high quality linings, I would try a set of the better ones (a relatively inexpensive and simple fix if that turned out to be the problem). Before that, though, make sure that your adjustment is good so that you don't have excessive heat from dragging brakes.
My troubleshooting method is to start with the easiest and work toward the more complex, if I stray from that approach I usually regret it.