2002 Chevy Tracker Brake Pedal sticks

I currently live in alaska and that is where I bought the truck a couple of years ago, about a month ago i began to notice my brake pedal would stick when I pressed down on the brakes, and as the truck is run and heats up to operating temp the pedal begins to make a noise when I press down on it, it is not a squeking noise it is more like a clunking noise, and I can feel it in the pedal. I can easily pull it up with my foot, so I had it taken in and they replaced the master cylinder, and no change. I lubed the pedal with a lubricant and still no change. any Ideas? next idea I have is the brake caliper piston.