I rediscovered this BB gun, I had from the mid 1960's. I hadn't fired it in years. Chronographed it @ 205 fps, while an occasional BB dribbles out of the barrel. Quite low, supposedly should be in the 350 fps range. I purchased a seal rebuild kit along with mainspring. Still 205 fps. Something is wrong here. I made up a test plunger to test compression in the tube. There is none. I don't feel any air coming out the barrel either, but there is vacuum when withdrawing the plunger. I don't get it. Where is the air going? There is some sort of pop off valve barrel cap, threaded to the rear of the barrel. Maybe this has something to do with it.
Can anyone have experience with this BB gun that can give me some guidance?