Braking feels soft and like it's not slowing?

Intermittent, but more often than not, braking feels soft and like it's not slowing the truck as abruptly as I would like. When it does this, I can correct it and get a much stiffer feel by pumping the brake once. Just once is enough to correct it, but it has to be a quick pump (less than a second off then back on, otherwise I have to do it again). About 2 years ago, I fully replaced the brake fluid and the rear drum cylinders, hardware, and shoes. About 1 year ago, I replaced the front rotors and pads with some upgraded drilled and slotted rotors and carbon fiber semi-metallic pads (PowerStop Z36 kit). The current braking issue started about 6 months ago. There are no brake warning lights, fluid holds at a steady level, and I don't hear any vacuum leaks.