67 Drum Brake Fade?

[Matt67C10]

I was sitting at a light yesterday after having driven about 30 minutes on the highway and light traffic. I was stopped on a slight upfhill with my foot on the brake when all of a sudden the pedal sinks, the brake light on the dash comes on and I start rolling backwards! I pumped the pedal a bit and stopped the truck, the pedal got firm again. I had no other problems on the way home. I checked the fluid level, it's full. I'm running the original, non-power drum setup all around. Is this just fade?