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,