If you haven't cured it already
Follow Neil the Wheels instructions for removing the chuck and maintenance.
Then each time you drill masonry make up a carboard cone large enough to fit around the chuck with the top end opening equal size to fit around the drill bit. tape or 'Blue Tack' the cone to the drill bit that will stop dust getting in the chuck it cured my dust problem. Or you could even use a small plastic funnel, shorten the spout end as required and fix as above.
bob
January 2005