I feel your pain on this, frustrating when a modification gives you so much trouble.
There are two seals that seal the torque converter apply and release fluid to the converter. The first one is on the end of the turbine shaft(shaft that sticks out of the front of the transmission). It is an o ring, usually black or orange. This should be replaced every time you take a converter off or on. If you cut this o ring you will have an internal leak in your TCC circuit.
#3 in the picture
turbine.jpg
The second one is inside the stator support, which is the housing the converter goes into. There is a blue seal in this housing that you cannot replace unless you disassemble the stator support. I have personally fucked this seal up once( I have built about 30 8L90's so far) when I didn't get the converter straight into the stator support.
#3 in this picture
stator.jpg
Not saying these are your issues but it could be the case.
I am surprised that Circle D did not include this seal or anything about it in instructions.