hi i also have a 99 200 mxc and the mastercylinder actually pushes air into the line. i have rebuilt it once already and i worked for about 2 months then same thing happened. i suggest buying a new mastercylinder. thats what i am in the process of doing. that rebuild kit is like 50 bucks from a ktm warehouse in canada( so i wouldnt have to pay shipping for over seas ) i dont know the part # but if you do decide on the kit make sure you clean the cylinder out very good with a cleaner of some sort( alchol or lacqure thinner) if there is any dirt in where the plunger is and you put the new kit in then youll have to get a nother kit very soon.