WOW - now the whole story comes out. It sounds like the OP' s trans was toast to begin with.

I wasn't trying to fool anyone. I just wanted to know what I should check to be sure whether the trans was gone or not. If I did all the things suggested and none of the worked, then it's probably a dead soldier.

Again, not trying to deceive anyone!

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85 300D

All things considered, sounds like Frank had you on the right track of checking the working pressure of the transmission, for details just use the search and see how others have adapted banjo bolts with a line and pressure guage to test-

Looks like the torque converter is just not getting fluid.

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85 300D

there are two possibilities then

the pump is dead or the pressure regulator for the converter fill is locked in bypass. (usually happens on honda though) - you can test this with an ATF cooler line going to the radiator, no fluid means nothing going to converter.

the other is that the filter you installed is not tightened or the cork gasket fell off. The filter once tightened up should never be loosened unless you are replacing it. The cork gasket crushes to make the seal. Or the filter maybe blocked internally.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

R.I.P transmission

That reminds me of a marine transmission that I changed the filter on once. I didn't get that gasket in just right and the transmission sucked air and wouldn't build any pressure. Way different kind of transmission, though.