I did in a valve seat on a streamline dry firing it. Probably what is wrong with yours. Not a big part and it’s cheap. Best to slightly open the reg up to let the plenum drain rather than dry fire.
Well here I’m again feeling really stupid. what I learned! don’t mess with you airgun before ask the profesionals. Here is the story I replace the white plastic disc on the huma regulator pushed the stem rod back Then I pressurize the gun shot a few rounds and it works really nice. I lost one bolt please if anyone knows the the size, please let me know. I really appreciate.