Weak disc brakes on P-30s is nothing new. My '93 will heat the brakes up if I do not slow down at the top of the down grade, shift into a lower gear sometimes even low to descend.
Better brake fluid helps, and the pads should be severe duty ceramic(SX149 on mine) and not the standard pads.
Going down the grades, the engine exhaust should be cooling rather quickly. There will or should not normally be much heat transfer to the brake lines.
You could have other issues. If it has happened a couple times, the rubber brake lines could be collapsed on the inside restricting fluid flow and return.