Vacuum pump on the motor that supplies vacuum to the brake booster. We have replaced this pump (Isuzu PART #: 97241035) on this vehicle 3 times in less than a week. These parts were Isuzu brand and the exact part number for our VIN and application. Went to hit the brakes, the brakes were not working and extremely hard to press, and not slowing the truck down. Driver ran into another vehicle. Yes, the problem has been reproduced multiple times by our internal tech and local Isuzu dealer. Also can be found on different forums with exact issue (see attached). Each time the part has failed, we have sent it back to dealer. No prior warning until the brakes were applied and not working.