1999 Gmc Sierra Truck Excellent Condition Havent Had Issues Until Last Week

My truck is in excellent condition havent had any issues until last week when i was trying to stop. I had no issue slowing down but when i was just hardly moving the brakes starting to slide and it felt like the pedal was pushing back up. It has happened numerous time since then never while going fast just when im slowly coming to a stop that last few km/h i slide. I am also hearing some noises when the sliding happens. We had the brakes apart last week and they are in great condition what else could be causing this? I really appreciate you taking the time to read this and would love to hear your feed back.
Thanks Mitch