If its cutting out when you hit the brakes then you need to adjust your throttle set screw. First put yoor throttle trim right in the middle. Then remover your air filter and see how far open your carb is. If its 1-1.5mm open then your good. If not adjust the idle screw till you see it open about that much. Now hit the breaks, dose the carb close all the way? If so thats the problem. You may need to take the servo horn off the servo and let that center also. When you hit the breaks the carb should still have that 1-1.5mm gap.