I'm assuming your truck is all stock. If not, disregard all of this. I had a similar problem with my 47 when I started working on the brakes although all 4 corners were dragging on mine. it turned out the relief port in the master cylinder was plugged so the brake fluid couldn't return to the master cylinder when you applied the brakes and got off the pedal. It would improve if you bled the brakes until you have enough pressure to lock them back up. Also make sure your pedal is adjusted properly and the brake return spring is installed so the cups uncover the relief port when you let off the brakes. Hope this helps!
Best regards,
Les
