Daisy Daisy 853 troubleshooting question

I had a SSP pistol that did a similar sort of thing. The breach seal looked chewed, the piston seal looked fine. It turned out to be the piston seal. I figured it out by changing all the seals one at a time. Fortunately it was the 3rd seal I reached so not a huge tear down. How old is it? A fresh set of seals might be in order. I have a 753s and would miss mine
 
first off if you are going to take a gun apart that has a problem replace all that you can even if it looks fine
i have done enough 7-7 pistols to just replace the old
and i know you were just hoping
i got my 853 out today and gave it a few pellets and it worked fine but i was looking to refresh my memory if anything
when i got me 853 it was used in restored condition and shot sort of slow and i set it aside
what i found was there is a small plug in the air chamber to the valve and when you pulled the trigger it leaked all or enough of the pressure to make it under preform the plug is not a part and i installed a grub screw
do i think that is the problem No
tell me after firing the gun is there any air left meaning will it kick the handle forward or is it empty
i think it is leaking and be the time you get to pulling the trigger there is just a little air left and you get the sound in the video
is it a guess, it is but these are a put air in pull trigger dump air out rifle

new parts and test is all you can do