I developed a leak in my Mishimoto after a few months too. It was actually coming from the driver's side. I have subframes, but they aren't full length, plus I have a vert, which means more chassis flex. Instead of returning the radiator and having it happen all over again, I used Barr's liquid aluminum stop leak. I was wary of using it, but it was guarenteed to work and not clog up my system. So far so good. It doesn't leak anymore, my engine temp is fine. I would stay away from the older stop leak products, but some of the new ones seem better.