The brake peddle went solid hard and loss of braking happened while backing a boat trailer down a boat launch ramp. When the brakes failed the dash flashed service brake assist. Loss of power brakes caused the truck to not stop even with pushing down on the peddle as hard as possible. This occurred while coasting in reverse off of the throttle using the brake to slow the vehicle and boat down while going down the launch ramp in reverse. Speed was approximately 5 mph. I managed to stop the vehicle at the water line by applying the emergency brake. The local Chevrolet dealership where the vehicle was purchased replaced the vacuum pump under warranty due to weak vacuum. I lost power brakes again on the way home from the dealership when backing down an incline on my driveway at low speed. The dash flashed service brake assist again. Returned truck to dealership the next morning and took a technician on a test drive with me. Showed technician loss of power brakes and service brake assist message on dash when braking in reverse on an incline. Chevrolet dealership says they contacted Chevrolet technical support and replaced brake booster and replaced vacuum pump for the second time. This is on a 2018 Chevrolet Silverado 1500 LT1 with the 6.2L engine and 42,155 miles.

- Holiday Island, AR, USA