If anyone has this issue of lost brake pressure at idle or stop(low RPM) and or hard brake pedal. I had a feeling it was the brake booster pump. Took to local dealership. They verified that it was the problem. My 2017 Silverado is out of warranty(47k miles) and thought this was gonna cost me $$$. There is currently a special warranty on this problem due to crash risk. Luckily it will cover the entire parts and labor. I do not know at this time if there is a mileage limitation on this special warranty, but if you're in the same boat as me, go get it checked out!