I had the same issue with my truck (front disk, rear drum). My research showed that this is not uncommon in installations where a new master cylinder is installed (generally dual replacing the stock single). Typically it was the front brakes not releasing fully. My problem was the dual master cylinder also. It took a while to find the right adjustment of the push rod, where I had good brake and the brakes would fully release after letting off the pedal. If your truck doesn't "roll" easily after using the brakes for a while, it could mean the wheel brake cylinders have not fully released. Possibly the the travel on the master cylinder push rod needs adjustment. In any case, my research showed that to be a common cause, and that solved my problem as well. Maybe something to look at if all else fails.
don
