Am I correct in saying that cavitation is caused by starvation and washed out is caused by something in the water, abrasive, chlorine, hard water, etc.
Remove the inlet and outlet valves, where the valves seat in the head you will see cavitation, the surface should be smooth, if it is pitted then you have cavitation or washout, take a light and look down into where the valves seat, it should be a smooth surface, you can check this by using a small screwdriver or your finger, it is obvious, the surface should be smooth, if not it is either cavitation or washed out, there will be pitting that you can see or feel with you finger. I just went through this b****, all the surfaces should be smooth, if not you have a problem with one or the other.