Why on earth would you use a heavy duty street pad on a track??
The K500's are not designed for track, the K750 "Trackday" pads are, hence the name.
The fact you even managed to get 2 trackdays out of a street pad, says alot for the pad.
Thats almost as intelligent as using Safeway engine oil in a V8 supercar and wondering why your engine blew up....