Should I argue with this mechanic about my car?

[Makavel]

So a couple of weeks ago, I went to the auto shop to get my car fixed for a antifreeze leak. They told me the leak was coming from the water pump, and replaced it (it cost me $285.00). And just one week later, I have noticed the same problem was still persisting (antifreeze leak). I took it back to the same mechanic, with the receipt of the first time I went there. They told me the leaking was coming from te radiator this time and that I had to replace it (around $300.00). And they wont do it for free, even though it is the same problem. I dont know whether I should argue with them, because the first time they didnt tell me nothing about the radiator, and I'm not even sure if the leaking was coming form the waterpump the first time I went there. I live in a college town, and I have been told that most busineses around here try to take advantage of college students like me.