When I replaced my radiator I decided to also flush the transmission. I drained the transmission and did a "EXACT" measurement on the amount of fluid that came out. I removed the top radiator line and refilled the transmission with the same amount of fluid. You fill the transmission via the "line" not going through the radiator. Make sure to replace the o ring to avoid leakage. When you drain the transmission you should get around 3 quarts of fluid.

 

 

Yep, gravity feed is enough it just requires a little patience. Remember to use the top line back to the transmission, do not go through the radiator. I get my O rings from NAPA but some hardware stores also have a good selection. Another issue is to make sure the fluid you are using meets specification LT 71141, I used Castrol.

The big issue is the problem of having the correct amount of fluid in the transmission. Unfortunately the only method of doing that is rather involved and requires having the car lifted and checking while the car is running and up to operating temperature. Pretty dangerous unless you have a way to safely lift the car , For me that was a job for a good shop familiar with ZF transmissions.
good luck