I am a forklift repair technician and a barb ward fan. Here are some possibilities to look into. Wheel cylinder plungers sticking, fix is honing the hot cylinder and rebuild them. I believe you can get a rebuild kit by the diameter of the cylinder. As previously mentioned choked or partially blocked line. Try blowing air through the line, and observe fluid coming out. The mechanical drum adjustment on that side may be a bit tight but I am skeptical of that if it feels correct then heats up. Check everything else and if it is all good adjust brakes then test drive and when hot back off a Tiny bit and leave adjustment there.