Bruno, the Clavietta, has 5 hole that let air out as part of its design. You can plug the holes but the sound will change a little. What I have done is tap and install some set screws that can be removed if you do not like what it does to the sound. personally it does not bother me at all. You will have to remove a few of the keys to get to them. If you have leaks on your hard washer that is different or if you are leaking from the air chamber both easy to fix. see the attached picture to see what the holes look like. if the washers are very brittle they will leak as well. you can replace them with rubber washers and that will seal it up nicely. There are some pictures of the washers on this thread as well. Also make sure that the (4) screws that hold the air chamber down are sealed too as air can leak from there. You can tighten the screws at the back of the keys and that will push the hard washers to the base plate helping stop air. But it will change the feel of the key if to tight. Hope this helps. send me a picture of both you Clavietta’s and I will compare them to the ones I have.
Hope this Helps