I'm guessing there's residual vacuum stored on board for heater valve actuation. At least it's true on most other makes -- this is a safety feature to supply assistance for power braking at least once should the engine kick out.

After all the old coolant is flushed out, drain as much water as possible by opening the radiator and block drains and siphoning the overflow tank. This is to ensure you have enough empty space for the antifreeze charge, since you can never get all the water out. Add antifreeze according to the specified capacity, i.e., if your system capacity is 10 quarts, add 5 quarts antifreeze for the proper 50-50 coolant mix. Top off the remaining capacity with water.

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95 E320 Cabriolet, 169K