After draining, close all the drains and fill the reservoir. There are two bolts that go down into the head just beside the valve cover to the right of the oil filler as viewed from the front. Open one of them so air is expelled from the block as it fills with coolant. The Haynes manual says to remove the rear bolt, but both seem to lead to the same chamber in my M103.

Sixto
91 300SE
81 300SD