The problem is that the replacement rubber components that are available are not designed to be used with DOT5, they are made for use with DOT3 or 4 glycol based fluids. DOT5 can destroy rubber parts that ate not made specifically for use with that type fluid. Also, while DOT5 will not attract water, it also will not disperse it like glycol based fluids will. That tends to allow any fluid in the system (remember, these trucks have vented master cylinders, moisture will get into them) to settle at the lowest points. That will not only allow for rust pitting, but can also lead to spongy/no brake pedal under baking conditions that get it hot enough to biol the water in the system.
As mentioned, I bought into the whole "DOT5 is better for old vehicles" thing, and did run it in my truck, which had a complete brake rebuild, with all new lines, master cylinder, wheel cylinders, and hoses. Took DOT about a year to ruin the cups in the wheel cylinder and master cylinder. I contacted the manufacturer, they told me that none of their products were rated for use with silicone fluid. Got the same response from all of the other brake parts manufacturers except for EIS, who is no longer in business. I have not had a problem since I went back to glycol fluid.
