A 77 40 has a parking brake that is a drum type brake mechanically connected to the transfer case output shaft--it is not connected to the hydraulic brakes in any fashion
More research reveals you have a good point--the dual master on the '77 pushes fluid in two directions--the trick is to get the fluid going to the front discs and the rear cyls in equal proportions--maybe that is the reason for the prop valve--hadn't thought of it in this situation before--just...
I am not sure of this "volume" issue. If the master has a full reservoir, and the master stroke pushes -for example 10 cc''s of fluid into the system, the wheel cylinders should move 10ccs of fluid(front/rear--if they don't the wheel cylinders have a volume greater than the master--This would...