I think that every person who has a W126 has been down that road. When you look at the reservoir from the driver's side of the vehicle, it remains full. The front reservoir wraps around the drivers's side and obscures the rear reservoir.

To see the rear reservoir, you must lean over and view it from the passenger side of the vehicle.

Additionally, the rear reservoir holds very little fluid. You can easily run it dry if you press on the pedal 15 times.