2004 mini Cooper coolant issues
 

My brother has an 04 mini Cooper that started boiling water in the overflow. We changed the thermostat and looked at the housing real good. It appears to be good so we opened up the bleeders and started filling it with coolant. Noticed it was taking it awfully slow. Closed the lower bleeder and then started it up and tried to put more coolant in it. It just started boiling over almost immediately. We never got fluid to the top bleeder and it wouldn't take any more coolant. Clogged system? And if so is there a common place to start to look
Thanks john