firstly try the breech seal ...a bit of tissue moistened with oil or whatever placed over breech while you fire a pellet. If leaky it will blow off.
If breech is fine, consider trying O rings in place of the original rings if you do not want to buy originals. I replaced the rings on an Osprey with O rings and it was fine ... my notes say BS116. I believe they are the same for the Hawk.
Cheers, Phil