Just a thought on converting to a dual circuit brake system. Easier said than done. F600s usually have a single frame mounted booster, so just replacing the master cylinder with a dual MC will not do it. I have seen mediums from the late 70s with single circuit brakes. And I do not know if a light truck style dual circuit MC with a dash mounted booster will have enough volume for the dual cylinder rear brakes

One way to get dual circuit brakes is to get a dual circuit MC and use it with two separate boosters as was done on some of the 70s B series school busses.