C 60 brake questions?

[old Chevy guy]

My neighbor Has what I believe is a 68 C60 Dump truck. It has only a single one line master cylinder. He has been asking me how to upgrade to a duel system for safety. Has anyone any info on brakes on these bigger trucks that can give me some insight on what it needs? I'm really surprised that it doesn't have a double system already or should it? Maybe somebody else changed it over to older parts? Let me know what other info you might need and Ill go over and look it over.