The block drain valve (which is not a bleed valve) is under the rear exhaust manifold and can be opened with a crescent wrench. Attach a hose to the nipple for spill free draining. The radiator drain also has a nipple for a drain hose.

Daimler Benz made it very easy to change coolant on the M103 engine.

You can also flush the system by removing the heater supply hose at the left rear of the head to flush both the block and heater circuit.

Duke