Nick, 2 simple checks, have you got air at palm coupling 21 and are the brake actuators 11 moving on axle 2?? If the answer to both is no then the most likely scenario is tank 6 has no pressure, suspect regulator 15.
If you have no pressure in tank 6 it will also prevent the inter axle diff and front wheel drive actuators from working, it is vital to lock the inter axle diff if going of road as it will seize up if you spin it for long enough.
Let us know how you get on.