[chris vickery]

I had a similar problem on my first 12 cab when I had just fully rebuilt the braking system.
My pedal had little free play and the brakes worked great for a little while. After a couple miles of driving the brakes would stick on and the drums were heating up.
Investigation reveal led improper adjustment of the pushrod which was not allowing the fluid to release freely back into the master cylinder. As the fluid got hotter and continued to expand the result was application of the brakes by sheer hydraulic pressure buildup.
Once the pushrod was adjusted the brakes worked perfectly.