I had the exact same problem with my 49'. My first thought was my master cylinder wasn't releasing back. Turned out to be the front wheels cylinders weren't moving freely from lack of be driven. A combination of some rust and brake dust oil/sludge buildup. They would expand out upon braking but not return back. If I let the truck sit for a bit all was fine as they eventually crept back in until I drove it again and tada the fronts would lock up. I took the wheel cylinders apart and cleaned the rust and crud off of the cups, a little WD40, and all was good again.
