The M103 has a couple of bleed ports where the valve cover meets the head near the dipstick handle. A 14mm sockets gets them out. I think you only have to remove one since they lead to the same passage. Keep track of the copper crush washers.
Given the reverse p-trap configuration of the lower hose (in a W126 anyway), I pump the highest point of the lower hose repeatedly while filling the cooling system.
Check your reservoir cap as has been suggested. If it sticks to the neck when you remove it, something's not right.