I have the same kit (5/8 though) and had the same problem, but with the upper fitting. Discovered something I had not known when I first installed them back in like 2012. And here it is:A couple of year ago I installed a set of Red's Auto Rehab 1/2" transmission lines in my 2006 2500HD. It worked fine for a while and then started leaking at the connection at the lower radiator. I took it apart today and it was very loose where the 45 degree "adapter" screws into the cooler inside the radiator. Assuming that was the problem I tightened it up as much as I dared (I can't get a torque wrench on it), then reassembled and took it for a drive. It's still leaking just as bad as it was before. Below is what the set looks like along with a close-up of the offending connection (this is from their website, mine is still on the truck).
The O-rings do not appear damaged. It's really hard to see down there but I'm assuming the connection pushed into the cooler and the O-rings seal between the line and the cooler and that the face of the cooler is compressed against and seals on the inside of the radiator. Is this correct?
So what could cause this to leak if the O-rings aren't damaged? It is not leaking water, just transmission fluid. I can replace the O-rings if that's the problem but it's a fairly long operation so I want to be reasonably sure that will work before I start (just letting the truck idle won't cause it to leak, I have to re-assemble everything and drive it a bit).
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Install 2 o-rings back to back at each of those fittings that go into the cooler. One o-ring will provide a back stop for the other taking up the slack that is causing the leak. Do one fitting at a time or the cooler could fall inside the radiator.
I know, sounds stupid. But look at the fitting pic above and you will notice that very little of the o-ring sticks past the washer. Most of it sits down inside the washer. You can't use a smaller ID washer either as it won't slide down over the threads. So, just use 2 o-rings. Go for Viton material if you can. Been maybe 4-6 years since I did that and no problems since.
I tried calling Red's years back when this happened and could get no one. Found the instructions for a similar kit and that's where they actually instruct by design to use 2 o-rings per fitting.