i've let out all the fluid and put a new filter in and gasket.
How much transmission fluid do i have to put back in to fill it up the way it should be.

cheers

1975 FSM says 17 pints, 8.5 quarts for a 904 but that includes the torque converter. This assumes a stock pan.

I believe the converter is 4 quarts on its own. If you didn't drain the converter, I say add 3-3.5 more quarts so you know you aren't over capacity. Start it up, get it warm then run it through all the gears. Check the dipstick level in neutral on a level surface and go from there.

Anybody know where the neutral operating level should be in comparision to the top of the pan?

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Cheers!
Al T

Al - take the dipstick out then place it alongside the tube at the same height as if it were in the tube. Then you can look at where the mark is on the stick.