928 S4 Automatic 1989 Transmission Leaking - Correct Reservoir Level

Hi, My Transmission has been leaking for a while, due to personal circumstances, I have not been able to get to drive the car. I worked out that it lost about 1 litre (kept a tray underneath the car). Before I look to top the transmission fluid, it is worth mentioning what my levels are currently. 

1. Leaving the car for two months, Car not started, level is over the Maximum Line.
2. Start Car, wait and then move through gears. The level whilst the car is running is right at the bottom (see picture).
3. Turn Car off, check fluid level and the level is exactly at the cold 20 degree mark (see picture).
4. If I add what I assume has been lost, i.e. 1 litre, I would assume that when the car starts and is running after moving through gears, it will be at the 20 degree mark
5. However, I would also deduce that when I turn the car off, the level would go over the maximum mark. I understand the difference between the Min and Max mark is 0.2 litres.

I am assuming that I need to put in the 1 litre, or enough transmission fluid until the level comes up to the 20 degree mark when the car is running and disregard the level after the car has been turned off.Thanks

Porsche 928 S4 1989 Automatic LHD

 

I've attached pages from the Workshop Manual to check transmission fluid levels.  In short:  engine should be on and idling in neutral per the workshop manual (I prefer Park for safety)

Hi thanks, that is really helpful.

In trying to pinpoint where the leak is coming from, my initial thought is the B2 Brake Band, however before I put the car in storage, I had the Reservoir, Bowden Cable O Ring and breather hose replaced, as the leak has been persistent for some time. I noted that the leak after the work was done, was more excessive, and another train of thought is the breather cable. I could not find the part number for this, and the garage installed a pipe from their parts bin. Someone mentioned that the wrong type of material in the breather pipe, if it was not resistant to ATF, would swell up and cause a fluid leak. For the life of me, I can't locate the part for the breather pipe that connects from the top of the reservior to the transmission (see picture attached). I understand that this is a 3mm diameter inner and a 10mm diameter outer pipe.

Any ideas on a part number for this?
 

Doing a quick search and finding on another forum (not sure if allowed to post here), it looks like that house should have about a 5.5mm inside diameter, and believe that to be Porsche hose part number is 999 181 167 50. 

Pelican Parts sells it and from the aforementioned forum, it appears about 6" is needed for the vent hose?  www.pelicanparts.com/More_Info/999181167...n=999-181-167-50-OEM

Fantastic, thank you.

I have ordered the part, so I will know in a couple of days if it is a correct match.