Well, this one gave up the ghost!
It actually held up about as long as expected
Leather seal on the left from a Girandoni rifle after going through over 5 boxes of round balls.
Valve stem and seal are from a spare valve.
As it started to fail, it was a learning curve on the guns power. When it was in great condition, the gun was not very powerful... it was in the mid to low 400fps range. But as it got worse, the stem was higher in the valve face. Resulting in velocities near 600fps. Huge difference with very little length change. So it was either that or the dying seal was letting more air out around it.
Regardless, a better seal will make the gun a decent rifle.