or, on almost all controllers, there is a slide lever that can actuate the brakes on the trailer without hitting the brake peddle, just slide the lever when driving and you can feel the trailer brakes.
That.
At a very low speed, nothing in the way, with the trailer break control power set to mid level, just reach down to your proportional controller and activate the manual break engagement finger lever device thing, slowly, and see if you can feel them. Be prepared to use them but, no foot break for this test. Adjust power accordingly. I want the trailer breaks to be able to stop me and the trailer at low speeds but not necessarily HARD.
Trailer breaks, to me, aren't as much for stopping as they are for helping to stop, keeping the trailer settled. I don't really want to feel them much once you're all set and going because I don't want them to be working too hard thus prematurely wearing. Let the truck do most of the work and enjoy the confidence that the trailer breaks are helping.