Last time I had a similar problem with a truck, the trouble was caused by the flexible brake lines swelling on the inside. The swelling on the inside can be overcome by the pressure frome the master cylinder, however the amount of restriction inside the lines is not easily overcome by the brake fluid attempting to return toward the master cylinder. These TRs are now about 20 years old.
I put braided stainless on my Ttype when I installed the Baer 12" brakes.