When you disassemble the tranny you must also remove the brake disk. With the brake disk removed there is nothing to keep the caliper against the piston to lock the blue brake post in place.
You will have to pull the brass piston out of the tranny housing slightly so that the post can be fully seated and the slot in the post can engage the piston.
Now would be a good time to add springs between the calipers (ink pen spring cut to size). Not only will the springs prevent brake drag but they will also keep the caliper pushed against the piston and the post will not fall out if you remove the tranny at a later date.
