The brake linkage is connected to the same servo as your throttle, if you're looking at the top of your GST, you'll see two little rods connecting the throttle/brake servo to the brake cams (little arms that twist). When brakes are applies your servo pushes against those arms and they apply the brakes which are under your truck. If you look on the bottom of your GST you'll see the plastic front drive shaft, and metal rear drive shaft, you should see the brakes themselves mounted on each of the drive shafts, they look like 2 little metal discs on each shaft.

I hope that helps. I would post a pic, but I'm at work