Fred,
The Hydrovac is a Power Brake Booster, it adds an additional 900 pounds of system pressure which means the human doesn't have to push as hard to stop the vehicle.
When swapping from a single pot to a dual pot MC you should stay with the same bore size (your bus could be 1-1/4" or 1-1/2" bore). It takes a lot of fluid to expand the wheel cylinders, so if you use a smaller bore MC the pedal has to travel further to get the correct amount of fluid to the wheels. This translates to the pedal being closer to the floor with less safety factor. The MC conversion kits sold by the vendors are for the 1/2 ton trucks, the bracket may work if you find a bigger MC.
My trucks all have stock brakes and I don't have any residual valves, I just keep the shoes adjusted.
You may need to install an adjustable proportioning valve to the rear disc brakes until you get the front drums replaced to keep the rears from locking up before the front.
The ECI bore may not be large enough for the big trucks.
Mike B
