That sounds like a tough problem. Is that a machined Tslot, or just a cast T slot? The best thing to do is to weld up the broken areas. You might be able to form a piece of graphite to slip into the Tslot as a backer for the welding puddle. Otherwise, you'll have to remill the Tslot. Use a machinable nickel alloy filler rod.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)