One of my favorites wasn't "holding" when I reached for it the other day. The check-valve was hissing and was obviously the culprit.
I have worked on the Prods in the past and coincidentally had a spare check-valve in the parts bin so I tore into the front end of the reservoir. When I looked at the failed valve, the o-ring appeared dry so I R&R the Foster style Crosman unit. It seems to be holding so I'll keep an eye on it overnight.
The spare was ordered after I received my guns and I will assume it is the latest iteration of the part. It is possible that it is not for the pistol, I don't recall which parts diagram I used to order the part from Crosman. They are not identical, the failed part has a straight walled cylinder with o-ring seal similar to a spool valve, which I rebuilt for future use and the spare is the more typical tappet style with o-ring seal on the bottom. I always enjoy getting these leaks fixed without too much effort. I have an AA 410 with a valve leak and doubt it will be this easy.
Top valve is the failed valve, bottom is the spare.
Why does the replacement look like brass? I would be leary about soft brass on constant connecting type high pressure fittings.
I photograph under soft LED lighting which adds a yellow tint. Sometimes I will edit the tint out but didn't feel like it for this photo. The part is a Crosman Marauder HPA part.