Any large vehicle (1 1/2 ton or bigger) would have come from the factory with a hydrovac, I'm pretty sure. If yours doesn't have one, it's probably been removed or disabled. That's deadly dangerous. Untold millions of commercial vehicles were driven with single line hydraulic brakes from the 1930s to the 1970s or later. There is absolutely nothing wrong with a properly maintained single line brake system with a remote booster. My grandparents' company ran tractor-trailers based on 1934 Chevy 1 1/2 ton trucks between Milwaukee or St. Louis to Nashville during the late 1930s, hauling beer, with mechanical brakes on the tractors and NO trailer brakes for many years before hydraulic brakes became popular. They eventually upgraded those trucks to vacuum-assisted mechanical brakes.
Doing major modifications to a commercial vehicle brake system could result in some massive liability issues in case of an accident. It would be much better to rebuild the entire brake system, including metal and flexible brake lines to original equipment condition, especially if you're getting paid for the hauling you're doing. People who modify brakes, suspension, or steering systems are an ambulance chaser's wet dream! My attorney brother has spent almost as much time trying those kinds of cases as I have spent fixing virtually everything that rolls, floats, or flies!
That booster I referenced was just an example- - - -there are new, or rebuilt units available from most professional level parts stores such as NAPA. Most hydrovac units have a metal vacuum piston with rubber O rings, running in a large diameter metal cylinder.
Jerry
