Try tightening the brake linkage. there's a thumbscrew on the brake linkage. with the throttle set like in the pic there should be little to no slack in that link. the little piece of tubing should just be starting to pull the brakes on. you could move the rear alum stop back a little to reduce the backward tension on the link to the carb. My brakes were really loose out of the box. if the servo has to pull to far back to actuate the brake it'll pull back too hard on the carb linkage. you could really tighten that link to the brake and when you release the thottle to idle the brakes will come on and stop or slow the truck.