I like the idea of the brake pads swelling in the heat as a possible, or even the brake housings growing in length during the time in the truck. Also check to make sure that the brake arms are not bent that could be causing the problem (though you have most certainly checked for bending/broken arms I'd assume). Something has either stretched or shrunk between the levers and the rims. I'd put my money on the housings myself - if there was stuff putting constant pressure on them while heated (trunk + summer) I could see more than a cm or more of stretch.