Instead of the conventional dual brake arrangement where the front discs are on one system, and the rear drums are on another master cylinder chamber, several cars in the mid-1980's went to diagonal braking. On cars, not necessarily trucks, up to 90% of the stopping effort is handled by front brakes, due to weight transfer toward the front during normal driving. Rear brakes on automobiles are just there for stability, to prevent the rear end from trying to pass the front on a hard stop. That's why rear brakes, even disc setups, are so small and ineffective, to prevent rear wheel lockup.
On cargo-carrying vehicles, the rear brakes must be big enough to handle a max payload, but there needs to be a proportioning valve to cut back on the brake application on a lightly-loaded rig. Most of the time, it's a simple squat valve between the rear axle housing and the frame that responds to suspension deflection. Heavier load, more spring deflection, more brake effect!
Since the rear brakes on a medium-sized truck will probably have larger wheel cylinders in the rear than in the front, it makes sense to me to split the power boost (and fluid displacement) equally between both hydrovacs by connecting one front wheel and one rear to the same booster. Do it on opposite sides of the vehicle, and any tendency to pull in case of a failure will be minimized if one system runs dry. If you do that, try to use a master cylinder with two fluid reservoirs the same size, like the ones used on dual-brake, 4-drum systems in the early 1960's.
Jerry
