Tranmission line won't stop leaking! I replaced both lines they were leaking where the line's hose ends push lock onto the fittings in the cooler/radiator. Now one of the new lines is leaking. Does anyone know what size pipe thread the fitting in the cooler is. I think its metric. I'm going to make a new hose to screw directly into the cooler and flange the hard line portion of the transmission line. Off the subject why is there a check valve on the return line from the transmission besides the obvious reasons of backflow or back pressure.

 

It's AN threads, I believe, not metric.
-4AN, I think. Somewhere around there.

And your exactly correct: the check-valve is there for... prevent backflow of fluid.