Now is the little black piece under the screen just another superfine screen or is it a valve of some type?
Nitrile check valve, you can just replace the whole assembly if you want to. They’re prone to fail vac test, being 30+ years old.
From a post 6 pages back “This one “looked“ good, but failed vacuum as it was hardened up, so I just replaced it...comes as an assembly #86-523 so you don’t have to try to put the screen & that tiny midget clip back in.”