300E transmission O-ring
 

Did a quick search but was unable to find out how to get at and replace the fairly large o-ring (3 inch diameter) seemingly inside a metal cover plate on the side of my 88 300E auto transmission. There's a good sized slot in the cover plate - does it simply pop out with a pry?

On the front pump? If you refer to my transmission thread here you may find a picture of where it sits and I can help you further:
http://www.peachparts.com/shopforum/tech-help/271601-722-3-transmission-removal.html

This one right here indicated with red from your pictures.

http://www.peachparts.com/shopforum/...pictureid=2253

Just a retake. Still wondering how to replace the o-ring indicated in the picture above.