I believe the upper line is the pressure feed to the cooler, but a check valve isn't likely to help. It would be simpler and more effective to drop the valve body and fix it right. There's a "converter anti-drainback valve" that's supposed to keep the converter filled, so a check valve shouldn't be necessary. He can also stop the leak by plugging up the tiny hole in the middle of the driveline slip yoke rear plug. That's where the fluid gets out if the converter drains back into the case.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!