If that's your car pictured in the avatar ( Nada spec ) then my guess is you have a 9FU BW35 fitted, they do have a check valve spring in the valve body but the spring is very weak and just not up to the job especially if it has compressed over time as it's a really flimsy one.
Early BW35,s had a much sturdier spring which is fitted in a different location, and they also had a paddle flap fitted.
There is a check valve that you can fit in the cooler circuit ( return ) that should stop converter drain back.
Trouble is finding one ....
As for your leaks, they can be tamed to stop leaking, the main cause is the pan gasket / pan itself, the other main culprits are the selector seal and inhibit switch ( if it's from the switch you need to look at your fluid level ) then there is the pump seal, tail gasket,tail seal, pinion seal, ...........
I have also found on several late BW35,s the actual case casting to be flawed in a certain location causing leaks through a crack when in use. I though at first it was a one off, after seeing five the same I am certain the cast mold was flawed and the aluminium used in the cast process to be poor quality compered to the early case casts which are really good.
