My reaction would be to strip it down replace the breech seal and remove all of the old grease. Whilst apart, re-soak the piston seal in some neatsfoot oil. Re-assemble with fresh grease and see how it shoots. They can shoot very nicely with the old leather piston seal, properly lubricated. The only other thing to consider is the remedial sealing of the cylinder / breech end plug brazing with some Loctite. Not all HW35's had this fault, but whilst apart, you may as well do it ('cause it'll be a pain to strip it all back down again to do it it you find it does have this fault).