Black pepper does work. As Tester mentioned, you only need a tablespoon’s worth.
Bar’s Leak is a slightly different animal. Too much of Bar’s Leak can cause radiator tubes to get clogged. I had this problem, and used the Bar’s Leak when the pepper stopped working. It stopped the leak for a month or so, but blocked up tubes in the radiator, and I had overheating problems next. It all came together when the overheating stressed the already-weakened headgasket, and finished it off. I had the radiator checked since it was out, and flow was reduced to 40% normal. Had to buy a new one. The ultrasonic bath could not clear it.