With his big tires, stock Monster GT needs a little help to stop quickly. To increase brake power you can one of those following tips
Replace the small spring on the brake lever with 3/4″ of fuel tubing. After installation, check your brake adjustment to prevent brake binding.
Install the Factory Robo disk brake. This brake disk is thicker and will resist to fuel, dirt, mud and water. Part number 25410
Install a stronger throttle/brake servo. If you replace the stock steering servo, use it for the brake, it develops 111oz of torque.
If you still use the stock receiver, do the 4th channel mod.