I have a 1990 500SL with 66k miles. I replaced the radiator (the plastic neck broke), overflow and expansion tanks (including cap), thermostat, thermostat housing and hoses. The car never runs over 90 degrees, but will sometimes blow out antifreeze via the overflow hose. Also, the antifreeze will drain from the over flow tank into the block while driving and will return once I shut off the engine. I tested for head gasket leaks using a gas-test kit. there was no presence of gases in the coolant. i was told I could have air in the system, how would I get it out? i do appreciate any help you could give me.