oops! i misidentified the parts i was working with! i should have said 505 and 510.
The leak you are hearing shouldnt be coming from that part... the only seal that makes is at the top of the exhaust valve, once you pull the trigger there is no seal below, it gets returned to a firing position by the spring in the assembly.
If you are hearing a leak in the trigger frame (behind the trigger) its much more likely to be one of the orings in the assembly found in figure 6c (page 6) the on/off that pushes the trigger forward.