Make sure the throttle trim on your radio is not turned all the way to one direction. Ideally, it should be in the middle. If that is not it, let check out the brake adjustments. With the top off and the motor off, turn on the electronics. Watch the servo pull the brake lever as you apply the brakes. Make sure the rod attached to the servo horn is not just slipping on the brake lever and not pulling it. If all looks good, you can adjust your brakes very easily. Make sure your radio trim is set to the middle. There is a metal stop on the rod that pulls the brakes on. In that stop is a small set screw. If you loosen the set screw, you can slide the stop a little bit (only do small adjustments at a time and test how it works) toward the servo. This will engage the brakes sooner. Tighten it back down and test it. If you want more brakes, move the stop a little more.

Edit - Added picture of what to adjust if nothing else fixes it. This will adjust the brakes.