When I was running a one man truck shop back in the late 1970's as a means of keeping from starving to death, back when a Southeast Asia veteran couldn't buy a job, everybody hated air/hydraulic brake systems. They were downright unreliable, and had a tendency to dramatic failures at the worst possible time. I can't imagine why anyone would install one on purpose. For the same amount of effort, and probably less expense, you could convert to a hydroboost system that uses the power steering pump to provide power braking. Some systems even have an electric pump that takes over in case of an engine stall that puts the P/S pump out of operation.
There's absolutely nothing wrong with a properly maintained hydrovac brake system. Millions of them ran untold mileage, stopping max loads (or more likely big overloads) for several decades. My grandfather hauled thousands of gallons of beer over poorly paved (or sometimes unpaved) 2 lane roads from St. Louis and Milwaukee to Nashville in the late 1930's, using a 34 Chevy dual wheel flatbed as a tractor, and a short trailer- - - -with vacuum boosted mechanical brakes on the tractor, and NO brakes on the trailer!
Jerry
