How do you drain the overflow tank? Isn't it easier to connect a hose to the radiator drain and collect coolant into a clean container? As you said, you only have to get coolant below the level of the aux pump. You can see the level in the reservoir.

Where is the aux pump in a W140? I don't remember seeing it in my 93. I think cars with separate rear ACC have yet another pump in the right fender cavity.

Sixto
87 300D