I think most manufactors recomend flushing the cooling system every so many thousands of miles and if this truck was using water/ well-water maybe it helped filter corrosion to prolong the life of the coolant or to remove the abrassive corrossion from the sysetem to reduce wear on the freeze plugs and such? Just thinking out loud or in font.
1971 Dodge D100
So what's wrong with setting beneath a single pull string incandescent light bulb writing angry letters?