The following 27 pictures show how to convert mechanical valves ( they look like a machined aluminum block – MB 15 & MB 30) that are used on most of the ZF gears to having a “troll valve” feature.
1) Totally clean everything externally before you attempt this—WD-40, a brush, and lots of compressed air seems to work best for us.
2) Notice on picture 11 & 12 This is a plug that need to be removed. On the earlier gears, the plug was a thread “allen type” set screw that was installed with Loctite.. It can be very difficult to remove—Use a first class a allen hex wrench and you may have to use heat.. Nothing will get hurt but if you strip out the allen hex, you’ll be into drilling it out.. If it comes down to that, we use a “left-hand drill” and do it up-side down in a vise.. Gravity helps keep the drilling out and then we vacuum.
The later gears are simple—Just screw something into the center that “catches” and pull it out.
The rest is easy other than be sure all the “O” rings are in place (picture 14) —Grease on everything works well for assemble.