I had a similar viewloader for my first gun maybe 6 or 7 years ago. It sounds like you may have an air leak or blockage (although you should hear a constant hissing if it's a leak)
Here's what to do:
1) disconnect the air tank
2) ***MAKE SURE THE GUN IS NOT PRESSURIZED***
3) field strip the gun, check for missing o-rings, or any debris lodged in the gun.
4) if you don't find either of the above problems, you may have to further disassemble the gun and check for the same stuff.
5) if you still don't find anything, it may be a blockage in the air line. You could take it off and try blowing through it to see if you can. If you cant, buy a correct sized replacement ( http://www.amazon.com/b?ie=UTF8&node=3488701
). If you can, I don't know what the problem is, and I suggest trying to return it to him for your money.
Make sure that when you reassemble the gun everything is oiled up and ready to use.
If you have any more questions, let me know. I take no responsibility though if you get hurt because you didn't depressurize the gun or whatever.