Air locks in cooling system

Hi please help, i have a vauxhall insignia with 25000 on it. The car was overheating due to the cooling system having an air lock in it. The water pump has been replaced, head gasket has been tested which had no fault, also the cooling system has been presure tested and no leak was found. Despite the coolant system having been repeatedly bled. There is still air getting into the coolant system. Plesse advise will be very grateful.