Thats ur carb. not being open enough. So ever time u hit the brakes, it closes the carb to far and the truck shots off. Get ur throttle trim set to 0 and then then turn ur throttle screw on the carb in a 1/4 turn in . Fire up the truck then hit the breaks. If it stay run n ur good. If not turn it one hr. at a time till it stays run n. check ur low end needle after that to make sure ur not to lean, try n to combinsate for the low idle. Hope that helps.