C6 drain back after installing shift kit

[dv7508]

Installed a intermediate band in C6 while still in the '67. Not that hard after removing valve body. Did a transgo shift improvement kit while had it apart. Said to drill hole in between circuits for "line to lube" to increase volume and pressure to torque converter for heavy towing. Four hr. shade tree "rebuild/patch" and tranny works perfect. Nice.firm shifts, no slippage. Now I have converter drain back. After truck sits overnight, will not take gear for 30 secs. until converter fills. Check atf level after sitting, way up on the stick with a little leakage up top, definatley drain back. Did not do this until I drilled the Transgo recommended hole. Will a check valve in return line fix??? Where can I get one??? Found several part #'s on internet, everyone out of stock.

[Mancar1]

No expert by any means on the C-6, (except removal and install. ), but I came across this part # that is said to be used to solve the drain back. FOTZ-7D174-A.
Hope it helps.
Keep on Trucking......John