Same thing happened to me today, Broken valve
Apparently this Is not common and It's like a one and a million chance If It does happen. So It appears that the valve cover Itself Is made from plastic and aluminium. If this was made with steal, It wouldn't break.
I've taken the sensor off and put on a rubber valve
The problem I have now Is getting that damn warning Icon off the dash.
I may have to make a thread about this.