First, what you mean by "fluid side" on MC? Do the big trucks in 68 have dual chamber Masters like small ones? Isn't there a Hydro-vac maybe? This is another master and a vacumm canister to assist the system, making them Power Brakes. If you have a dual system, then maybe the Proportioning valve is to one side, blocking the other and needs to be re-centered? This is done with a small button at end of valve. Valve has four lines to it and a wire to turn on the dash Brake light. usually mounted near master or from cross-member on front? Maybe you should get big truck manual, school bus might have odd safer type system and require diff procedures??
