This maintenance tip below is from the cox website:
http://www.coxmowers.com.au/?page_id=1308

I fitted a new cutter belt and the cutter brake won�t stop the cutter.

Cutter disengagement should be adjusted so that when the engine is started and the cutter is raised to full height there is no take up of the belt. The cutter brake will only stop the disc if the cutter belt disengagement is adjusted correctly. (Screw adjuster out for more disengagement). Belt may be stretched (replace).

Happy is he who penetrates the mystery of things.