Looking a bunch better than when you started for sure. My thinking on it not dumping all the air would be more of a hammer spring or sticky hammer still. Is the safety staying up? This could also cause some hammer drag. Just a couple of ideas. I rebuilt my dad's 317 awhile back and it shoots great. They are great little guns for sure.
Next time you tear it apart put the valve exhaust spring in a vice and compress it one time until all the coils touch each other. This should make it dump better. I had to do it with my '69 347 even after I installed a new hammer spring.
On the older Benjamin's there is supposed to be a spring in the pump linkage that helps hold the handle up. The 317 I did didn't have it, could have broken long ago. I adjusted the pump cup up to the face of the valve and it worked. There is a video on Tim's site showing how to install the spring.