Purge Coolant System? Only Getting Air
 

I purchaced an '03 Cooper S back in May and it's been great. About two weeks ago I noticed a coolant leak at the inlet side of the radiator where the neck bolts on. I took it apart and replaced the o-ring. About a week later same thing. I flushed the coolant and tried to purge the system but only getting air w/ very little to no coolant, and it ran for roughly an hour w/ the fan running the enitre time. The inlet side of the radiator gets pretty hot and I'm guessing that is why the o-ring is failing. Radiator doesn't appear to have any build up in it and the engine doesn't run hot. Coolant appears to be circulating thru the system but still getting air. Is this normal on a Mini? Are there any known issues with the wate pump impeller? I've read the issues with the drive gears but I don't have any noises to speculate that being the cause. I may be overly paranoid about it but don't want to take any chances and causing a more $$ly repair. Any ideas or suggestions will greatly be appreciated. Thanks