Try shooting it a couple of times as you gas it up. The hammer pushes the valve pin in (and hence, the valve open) and a couple of quick shots while gassing up will pull the hammer away from the valve long enough for the valve to seal.
The o-rings on the HPR and valve body are available in the plumbing section of your local harware store. Considering that it has sat in a box for 6 or 7 years, consider replacing all of the black (buna rings).
No time for love Doctor Jones!