Brakes Failed problems of the 2011 Mercedes Benz GLK350

We have a Mercedes Benz Glk350 2011. While neither of its brake warning (which includes warning for low brake fluid level) and "check brake pad wear" display messages is on, and there is no abnormal noise from the brakes, its brakes can still fail for a short time, while work most of the time. In two cases, while I was waiting at a red light, the brake failed and the car moved forward, then stopped before it hit the car in front of it. At first I thought maybe I didn't pressure the brake pedal hard enough. After an accident happened, I am sure it was the brake's problem. In one case, while I was backing the car down the not-too-deep driveway in my home in a sunny, dry day, the car kept moving even when I pressed the brake pedal. I was pretty sure I pressed the brake pedal because I was able to move the transmission from "r" to "p" position. Even when the transmission was in the "p" position and I pressed the brake pedal hard, the car still moved down the driveway just as if there were no brakes. I should emphasized I drove over one hundred of times on the same driveway, and when the brake worked, I could always stop the car on the driveway! to summarize, my Mercedes Benz Glk350 2011's brakes can fail to work even when there is no warning message and no warning noise.