Even though a slip cap is not under much pressure, it is usually enough to open an attempted glue repair. Part of the problem is how small is the surface available for glue, the cap being very fine there, quite apart from the difficulty of gluing hard rubber and the stresses in that position.
I have repaired a 3000 where after gluing, I put a couple of O-rings around the cap to provide some constant tension against any spreading effect. This seems to have been successful but is better suited to a workshop pen. The last time I looked the cap seemed tight without the O-rings but I am not betting on its longevity without them.
You could be successful with Loctite 480 if you also swage a ring firmly into the slot where it is missing.
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