Someone, who sounds like he knows his stuff, posts this, on page 9
"Get a good mechanic who understands the 500SL properly and who also understands the significance of air locks on the 129.066 engine.
This engine is the most tricky of 129 range to refill with new coolant apart from 600. Fill expansion tank up to top of clear plastic where it meets black plastic. Remove overflow pipe to lower tank. Fill up to underside of filler. Leave filler cap off. Run engine at high idle and squeeze top rad hose regularly. When engine starts to get hot, turn cabin heater up to max. Top up coolant if it falls below top of white plastic. Run for 20-30 mins when fully hot, blipping throttle to clear air locks. Turn off engine AND DO NOT REPLACE FILLER CAP UNTIL TEMP IS BELOW 60 deg C. Replace overflow pipe.
Driving a 500SL with an air lock inevitably leads to overheating and a blown head gasket!"