Well, doing a thorough coolant system flush requires emptying the system from the bottom of the radiator (not that hard to get to) and from the engine block (helps to have a pet orangutan). While refilling (slowly)it helps to massage the hoses to get rid of air pockets. The bleed valve, located just an inch from the radiator cap, with a "+" type screw head on it, should be opened during this. It says something like "ENFLUGTUNG" on it.