Admin Adrianmsmith @slidingmike photo identified this valve with the green arrow? The photo you just posted is a completely different valve, called the vent valve. On this you can get the indentation. This valve is closed by a spring and opens (or should open) at 11-13 bar.
Adrianmsmith Admin sorry Dave, not my original post...... Sorry for the confusion, I was just offering an explanation as to how the rubber could have deformed or lifted.
