Yes I have a hydro booster and dual circuit Master cylinder in my 56 6100 I believe it is much safer than the stock single circuit master.
I didn't build any of the system it was all put together by ABS Power Brake. I am sure that the mounting bracket is much like what Randy Domeck above offers. The bracket ABS provided mounts the master cylinder under the floor using the same holes in the frame that mounted the original and uses the factory pedal assembly. The hydro booster and master cylinder were sized appropriately from another more modern medium duty truck with the same sized brakes.
Wetwilly as far as your friends bus the first thing I would do is check the rubber hose as Grigg suggested above. If the hose is clear then I would check to see that the ports on the master cylinder are connected front to rear correctly. The port at the push rod end of the master cylinder has a smaller working area and should go to the front brakes as the total working area of those wheel cylinders is smaller than the rear. If they are connected the wrong way the front brakes may be getting full pressure and making the pedal hard before there is enough volume of fluid sent to the rear to actuate those brakes.
Hope this all helps both of you.
