I have a built-in Trailer Brake Controller (TBC) on my truck, and have to adjust the gain on the controller for each trailer depending on the weight. I'm assuming your TBC would be adjustable as well. From my manual, here's how to get that adjustment right:
GAIN
The GAIN setting is used to set the trailer brake control for the specific towing condition and should be changed as towing conditions change. Changes to towing conditions include trailer load, vehicle load, road conditions and weather.
Adjusting GAIN
NOTE: This should only be performed in a traffic free environment at speeds of approximately 20–25 mph (30–40 km/h).
- Make sure the trailer brakes are in good working condition, functioning normally and properly adjusted. See your trailer dealer if necessary.
- Hook up the trailer and make the electrical connections according to the trailer manufacturer's instructions.
- In a traffic-free environment, tow the trailer on a dry, level surface at a speed of 20–25 mph (30–40 km/h) and squeeze the manual brake control lever completely.
- If the trailer wheels lockup (indicated by squealing tires), reduce the GAIN setting; if the trailer wheels turn freely, increase the GAIN setting.
- Repeat steps 3 and 4 until the GAIN setting is at a point just below trailer wheel lockup. If towing a heavier trailer, trailer wheel lockup may not be attainable even with the maximum GAIN setting.
It's kinda fun.