trani fittings

I just installed a new trai oil cooler on my 99 Chevy blazer and I was not sure how much oil I should add since I lost some when I cut the lines and the cooler and new tubing has an unknown capacity. As a result I over filled the train with oil, next thing I know is that the oil return line started leaking at the junction where it inters the transmission. I am wondering, if I already did some unknown damage to my transmission that fast or that the problem is just a lose coupling problem. If it is a coupling problem anyone knows how to fix it? Any directions would help.
Thanks

If it is the line where it enters the trans it should be a 1/2" fitting, tighten it with a line wrench. They don't take much more fluid when you add one of those coolers. I don't add any until I start the vehicle and can see where it is on the stick, then I know how much to add.