"Tight" is (or at least should be) one of the main things that sets the Chromatic apart from the other kind, and air-tight issues are common in all Chromatics. Slide assemblies, reed gaps, valves and comb seals are the usual subjects. A good inspection will always show where the "gremlins" are.
MK's are quality instruments and are usually pretty tight. Lay the thing in a quarter (mouthpiece down of course) inna inch of water, push the button in a few times then give it a smack on your thigh, and play something. If it sounds tight afterwards (only temporarily) the problem is in the slide assembly. If not, it's either valves/windsavers, or reedplate/comb seal, or your reed gaps are waaaay too large