If the cooler lines are connected the way they always were why would it all the sudden require a check valve not to leak?  In my mind all the valve did way keep the fluid in the cooler instead of letting it drain back into the pan.  Depending on the quality of the check valve, it may have simply failed, and that's why the problem has resurfaced.

However, I don't consider this an actual fix.  The first thing you need to do is determine beyond doubt and guess where the fluid is coming from.  Its not getting out of the transmission by osmosis.  There are only so many places from which it could leak.  Most transmissions have a case vent at the top.  If this were the source there should be trans fluid down the sides of the case.  It could also be leaking from the dipstick tube, the speedo drive, or the pan.  I would start by thoroughly cleaning the transmission and then use the old powder test to find where the leak is coming from.  

If the anti drain back valve is in the valve body its doubtful it would be touched during a rebuild.  IME they just clean up the valve body and put it back in, especially if nothing was trashed inside the trans (nothing broken, no metal, etc.).  Rarely do they touch the individual components, and my guess is that the knowledge of what a valve body does and how to inspect and fix it is likely a rare commodity today, as all auto transmissions have been computer controlled for over 3 decades now.  Once solenoids took the place of all those valves troubleshooting changed drastically.  

I know that it sounds improbable but here is what recently happened. We just had a winter storm (in Texas) so the car was parked in the garage. I checked it a few times and no leak. Then after about 4 days it dropped a large puddle of fluid. It is not a consistent leak - it seems to happen all at once. I've done the powder spray thing and can't trace the leak. The filler tube area is dry. It seems to be coming from up top - maybe the vent? Why would it do that after sitting for days?
The old guy that checked out the tranny last year has worked on these things for 50 years and he told me at the time that he had replaced the spring and check ball as well. 
It is a conundrum!   
I think I'll take it back to the transmission guy and give him another shot at it. 

I can't see it coming from the vent after sitting for days.  Fluid coming out the vent IME occurs when something going to the cooler gets plugged allowing the fluid to back up inside the trans and come out the vent.  Obviously the engine has to be running (pump pumping) for this to happen.  

When you say you can't trace the leak with powder what do you mean?  Do you mean you can't pinpoint the source, or that nothing leaks across the powder?  

Does it still shift like its suppose to ?! Not slipping or grinding/growling ?
Just puke'in fluid ...
6sally6

I would flush out/blow through the cooler and cooler lines while the trans is out.