My driveway is on a slope. For context, my friend had previously been in an accident a few months prior where he said his brakes failed at a red light and he was hit in the rear right. Apparently his brake fluid was murky so that might've been the cause. Whatever it was it got fixed and nothing since. I was going to drive his car since he didn't feel well, as I put the car in reverse it starts rolling down my driveway and there is no resistance when I'm holding on the brakes. I panicked and starting pressing harder a few times and even slammed them. It took until the 3rd slam for the brakes to work and I almost tapped my neighbors car on the other side of my driveway. They checked his brakes later and apparently nothing was wrong with them and they've been fine since. Just thought it was worth mentioning as now I'm always cautious about his brakes.