"Hydrovac" is a generic name for a frame-mounted power brake booster, which virtually all big rigs were equipped with at the factory. For instance:
www.ebay.com/itm/296905636303?It will be mounted somewhere behind the cab, under the bed. It uses engine vacuum to apply the brakes much more forcefully than the driver can do with foot pressure alone. The single chamber master cylinder provides the pressure to trigger the vacuum boost. In case of a loss of engine vacuum, the brake system still works, but without power assist. Any attempt to modify the brake system on a commercial vehicle hauling heavy loads should be made by someone who is thoroughly familiar with the physics involved. I've been an ASE certified master heavy truck technician for over 40 years, and I would be reluctant to attempt the modifications you're considering.
Jerry
