For the LS8-15 the inner tanks are filled first. For all LS8-18 versions the outer tanks need to be filled first for structural reasons. The the 18m retrofit actually includes changing the tank sequence (new pages in the manual and the lever sequence needs to be changed for dumping in the correct order).

I completed the TM 8006. At first it seemed impossible to work through the dumping valve. I mounted a lightbulb inside the wing, used a dentist mirror an some stiff wires to work. In the ende, with a bit of patience, sealing the crossfeed was actually not that difficult.