The trucks were designed to stop roughly 3000 lbs of unbraked trailer. This can be extrapolated from state laws about maximum trailer weight allowed without braking systems. The trailer brakes are supposed to stop the trailer on heavier models.
If you want to argue about equipment failure and having to slow down a big trailer with no brakes, you could be held negligent for not maintaining your safety equipment regardless whether you are over or under your GCWR or GVWR.
You may need a particular class driver's license and pay appropriate tag fees for your weight based on what you tow, but they do not dictate what equipment you must use.