1996 Buick Skylark

I had the water jacket housing replaced (themostst housing) which does not house the themostat. the garage that replaced it keep getting after market parts and they all leaked.(All 7 of them) Use the dealer part because it worked. My problem is that ever since they replaced that part the fan sometimes will run and does not want to stop. They told me that it has air in the system and since this model does not have a bleeder on it that you have to let it run with the radiator cap off. The garage supposedly drained all the coolant and replaced it and bled the air off. About every week or so I am having to turn the car off and run it with the cap off to get the fan to qiut running. the car does not over heat it just gets hot enough to make the fan run and Most of the time it works normal. Does anyone have any thoughts on what is going on with this?