If you can get a aftermarket valve stem with poppet from Will Piatt it will be a lot more durable. Factory poppets just don't last long. I'd also recommend you remove the hammer..... Check for metal particles in the grease... Clean tube and hammer and lube with molybdenum grease.
The reason I don't like to use a mallet on the end caps is because the tubes are spot welded together.... Don't want to put unnecessary force on the welds.
Cool, the little beast is back to work
The one thing with those rifle you should never store them full charge for a long time, or this is exactly what happens,.....time to send her to Will Piatt