If I can't convince you to go back with the original equipment system, I'm sort of wasting my time. I really don't know of a dual chamber master cylinder with enough volume to effectively operate wheel cylinders the size of the ones you have on that rig, so whatever method of power boost you choose will be irrelevant. You will run out of pedal stroke long before all the brakes get applied. Untold thousands of those trucks hauled maximum loads for millions of miles, and I doubt if very many of them which were properly maintained had a catastrophic brake failure. The proper size hydrovac was designed to handle max-load situations and provide adequate stopping power. Reinventing the wheel seldom has good results.

Check into what a good civil suit lawyer will charge per hour to defend you when your Rube Goldberg brake system fails. A $400.00 hydrovac is cheap liability insurance.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!