Sometimes I think that I only post questions for others' amusement. ;-)
Thank-you, I discovered that by assuming that it couldn't be a problem and perhaps I wasn't seeing the trigger assembly's interference.
I replaced the plenum and reservoir rear O rings, having them exposed.
On reassembly, air was coming from the regulator (Huma) atmosphere vent port, so opened it up and replaced the rear O ring. The front seal is a "plastic" material for which I have ordered a replacement.
Is there a need for that "special material" or will a regular "O" ring suffice? It sealed with only the new "O" ring replaced.
Thank-you again for the response.