I decided that at 47000 km it would probably be a good idea to decarbonise the expansion chambers with the caustic soda method. Well, in the end result on the first chamber was satisfying but now there is water trapped somewhere in the chamber! No matter how I turn the chamber, it just doesn't come out.
I can remember that I have seen pictures of NSR250 expansion chambers cut in half and that they chambers have two concentrical shells with some space between them. So I have drilled two 1 mm holes in the outer shell in the hope that I would hit the cavity but of course I wasn't successful. There must be more than one cavity...

Has this happened to anyone who has also cleaned their expansion chambers using the same method? How did you solve this?

I should note that my NSR is in pieces, otherwise I would just put the chambers on the bike and go for a long ride and hope that the heat would drive the water out.