The Stovebolt Site Forums The Parking Lot Big Bolts '70 c50 master cyl

As Wrenchbender points out when the market changed to dual reservoir master cylinders on the larger medium duty trucks they used two hydro-vac units splitting the hydraulics between the front and rear brakes. As time marched on they later began using hydro-boost systems.
Either system could be found on a later truck of the same size and retro fitted on to your big bolt. Just be sure that the donor truck is drum and drum and that it has close to the same working area to its wheel cylinders as your truck. Some bracket fabrication and plumbing is all either will take. However with the use of the hydro-boost system if if you don't already have power steering you would also need to mount a power steering pump for the hydraulic power weather or not you add power steering too good luck.