Research the laws in your state, and make your own decision as to what they mean.
I did for my state, and several surrounding states. I came to the conclusion that in many states a towed passenger vehicle does not comply with the legal definition of "trailer", so trailer brake laws do not apply.
However, many (if not most) states have a Braking Performance Law in the State Code that closely mirrors the FMVSS regulations: You must be able to stop any combination of vehicles within xx number of feet from xx MPH on a level, dry, clean hard surface. (MCA 61-9-312 specifies within 40 feet from 20 MPH for any "combination of vehicles".) If you can do that, you are legal.
Pennsylvania, apparently, has no such regulation (I have been unable to find it, anyway), however a breakaway system is required (PA Title 75, Paragraph 4502).
There are non electric or electronic systems available (Ready Brute is one) that are purely mechanical and meet the requirements of all states.
By the way, California does have a Braking Performance Law that closely mirrors the FMVSS, and is very nearly the same as the Montana (MCA) code!