One more question.. what is the distance from the pivot of the brake pedal mounting to the connection of the master cylinder rod and what is the total length of the pedal lever? From the center of the pad to the center of the pivot mount..

The 10 inch of vacuum is a thorn for sure but maybe you can change the master cylinder rod connection and "pick-up" some mechanical advantage / leverage and it will only cost you some labor. If you have the room, you might also add length to the brake pedal..??..?? Even an inch will add to or increase "clamping pressure" of the calipers.