The item 35 you mention (part # 7103366220 ) is a static pressure valve, and according to my spare parts catalogue from 1985 two of them are used for 4x4 as well as 6x6 trucks.
They are identical thus interchangeable.
The valves are displayed on the page for the master brake cylinder; I posted the piping layout to visualize my previous explanation.
The piping to the rear axle of the Austrian version contains a block numbered 130.02.04, this is the proportional adjustment for braking strength on the rear axle
Increased weight on the axle raises the brake force in order to prevent blocking the wheels
Details of that are shown here
Nevertheless we should focus on what is possible to diagnose in the jungle.
- you can swap brake drums and shoes from front to rear
- you can disconnect brake lines to verify how far pressure is built up towards the rear axle
- get yourself a manometer with an adapter for the brake lines and measure the pressure at all positions you can disconnect the plumbing