My quick test is when I get it all hooked up and start out of my neighborhood road. After I get up to a low speed I will use the SLIDER lever on the Electric Brakes and not the brake pedal. In my neighborhood I have a stop sign just a block away so this is great spot for me to test my stop just using the slider lever on the electric brakes...
This should bring you to a good stop which is only using the trailer electric brakes...
Some of the newer trucks with OEM Brake Controllers have everything tied into the truck computer and may not operate if you are under 10 MPH when using the Truck brake pedal to stop the trailer. This is why I am saying use the slider lever to do the trailer braking at slow speeds.
Obvisously the previous owner was using the trailer without working trailer brakes so I would figure out a way to see if everything is hooked-up properly.. The wiring to the trailer brakes may be disconnected or something.
You will be violating a DOT SAFETY LAW if you tow this trailer on public roadways without trailer brakes. Its kinda dangerous to do so too...
You can check with ETRAILER.COM and learn more about the electric brakes wiring with connector pin-outs etc...
Roy Ken