There is no mix and match with braking. There is nothing wrong with a hydrovac. Our gasoline tank wagons in the 1950s all had a hydrovac and they would stop like air brakes. There is no reason to have a dual master cylinder. If you have a hydrovac on the rear only the rear whels will lock up and you will switch ends before you know what happened. Just get some oil seals, rebuild your wheel and master cylinder, have your hydrovac rebuilt and your shoes relined, replace hoses and inspect or replace lines and you will have brakes that work. My father started in the gasoline business in the 1930's and I grew up with gasoline tank wagons. I bought my first semi in 1964 and I still own a dozen semis. I have learned a lot in these years. Brakes worked fine 65 years ago on 1 1/2 ton trucks and they still will if you don't screw them up with crazy ideas.
