We had one break at the school. Ever since, I've been very conscious of it, and warn my staff to always set the depth on the second half of a thru mortise to be just below the break-through point. It still kind of "slams" when it breaks thru, but without as much momentum.

http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/makita/7104L
It looks like part #44 can be had for $118 + shipping, but it would be several hours of labor to swap it out. CB.