Bleeding coolong system
 

I have a 1994 325i BMW with 125K miles. Over the years when working on the cooling system I always have had problems bleeding the cooling system. Just adding coolant to the reservoir is a very repetitive process. If you take it to a shop with air in the system they will diagnose the problem as a blown head gasket ($2500) (been there). Here is what I found works the first time. Open the cap. Disconnect the upper radiator hose at the radiator. Fill the radiator with coolant by pouring it down the upper radiator hose until it over flows the open reservoir. A little may run out the upper radiator hose connection. Reconnect the hose, close the cap and run it till it gets hot. Let it cool and recheck fluid level. Refill if necessary to mark. Usually this will get all the air out the first time.