I have purchased a slightly used 2017 GT Slammer BMX for my son. The BMX is fine, no visible signs of use, tires are almost new, all complete. The only problem is that the rear brake lever is very hard to press for my 11 year old son. I can press it and lock the brakes but he cannot so the brake is pretty much useless. I have tried to do some adjustments, checked cables, etc and I do not see anything obviously wrong with the brake system. All appears to be fine. Is the gyro system significantly harder to press than single cable system? Any recommendation what else to check or adjust before I take it to the LBS and spend more money on it?