Just a quick and easy pump flow check. Run car, check trans fluid level in park, check in neutral, no difference, no flow. Now you can drop the pan check manual valve, look for metal in the pan and filter, or inspect the whole valve body if it helps you sleep at night....or just start dropping your tranny. Sorry about your bum tranny BTW
For those following this, I blew a direct drum lip seal in -25 degree winter temps once. No gears at all because there is no connection at all to the output shaft. An air pressure check(35 psi) per repair manual will find this, and can save you from pulling the whole trans possibly.
Leaking or blown lip seal in front clutch pack should lose third gear and reverse only. Leaking or blown lip seal in rear clutch pack should lose forward gears only. OP lost all forward and reverse gears.
Gil Younger...Mr Shift...has a 904 'no yo-yo'' kit ( IIRC its included in the shift kit now) designed to reduce if not stop converter drainback altogether. ITs a new spool valve.