Hy miller873, unfortunately the sixties are a time of change in the medium/ heavy truck hydraulic brake world. I found a part number for a dual system master cylinder which is listed for a 1968 C60, the part number is Raybestos MC36346. If you were to install this master cylinder you would either have to install another vacuum booster for the second brake system, or you could install a Hydroboost system on the firewall in front of the dual master cylinder. I am not sure if a combination valve would be required on a drum/drum brake system. If you have a medium/heavy truck wrecker near you he or she may be able to supply the parts for the conversion. The master cylinder is available from RockAuto for $205.89, hope that helps.