You may first need to check the actual brake drum setting of the trailer wheel brake. They should be manually adjustable the same as any wheel brake setup with the screwdriver star wheel setting. I adjusted my OFF-ROAD POPUP trailer single axle brake drums with a screwdriver to the point where the shoes just make very slight contact with the drum and the wheel still spins freely...
After you have done the manual setting then go with the 25MPH road test setting your brake controller voltage setting...

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I use my manual thumb lever knobs all the time to slow down my truck and trailer without using the truck brakes.
I definitely do this at the first stop I come to when first starting out to make my trailer brakes are working properly...
Roy Ken