Seneca/Sam Yang Seneca Dragonfly MK 2 assemble/disassemble video???

PS
I thought, I gave some info on the DF II rifle, if I didn't, Sorry,
you can go to this place and on pg #17, Post 324, most of the info starts
there.
There are some photo's posted also, That site had several posts on the
DF I and II models.
From your info given, I think the internal valves are bad/stuck, or the "O" ring on the pump head is bad.
Hope this helps,