The first thing I would check is the brake line through the tongue. Make sure it does not have a slight kink, it only takes a small one to cause the brakes to not release fully.There could be other reasons, and someone should jump on here with things to look at, but I had the small kink and it did what you described. I was able to see it with a flashlight forward near the coupler, and a tug on the line freed it. The line was not folded over but was slightly compressed.
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