Thanks for your ideas. Brake pulling steering is a major safety issue on this thing so the front brakes need to be aligned together. The third wheel also has a drum brake operated by hand lever. There is a spoke lock on the rear but it is not effective because all the weight is on the front when parked, so the back just slides along.
I just remembered that there was a bend in the brake line between the housing and the brake (cable bent at angle), meaning there could be some extra slack. The shop didn't think much of it but now that I think about it, if there was less pull needed to engage the brake then it might line up with the pin. I'll be able to look closer and take a picture tonight.